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

Tgfbeta signaling directly induces Arf promoter remodeling by a mechanism involving Smads 2/3 and p38 MAPK

We have investigated how the Arf gene product, p19(Arf), is activated by Tgfβ during mouse embryo development to better understand how this important tumor suppressor is controlled. Taking advantage of new mouse models, we provide genetic evidence that Arf lies downstream of Tgfβ signaling in cells arising from the Wnt1-expressing neural crest and that the anti-proliferative effects of Tgfβ depend on Arf in vivo. Tgfβ1, -2, and -3 (but not BMP-2, another member of the Tgfβ superfamily) induce p19(Arf) expression in wild type mouse embryo fibroblasts (MEFs), and they enhance Arf promoter activity in Arf(lacZ/lacZ) MEFs. Application of chemical inhibitors of Smad-dependent and -independent pathways show that SB431542, a Tgfβ type I receptor (TβrI) inhibitor, and SB203580, a p38 MAPK inhibitor, impede Tgfβ2 induction of Arf. Genetic studies confirm the findings; transient knockdown of Smad2, Smad3, or p38 MAPK blunt Tgfβ2 effects, as does Cre recombinase treatment of Tgfbr2(fl/fl) MEFs to delete Tgfβ receptor II. Chromatin immunoprecipitation reveals that Tgfβ rapidly induces Smads 2/3 binding and histone H3 acetylation at genomic DNA proximal to Arf exon 1β. This is followed by increased RNA polymerase II binding and progressively increased Arf primary and mature transcripts from 24 through 72 h, indicating that increased transcription contributes to p19(Arf) increase. Last, Arf induction by oncogenic Ras depends on p38 MAPK but is independent of TβrI activation of Smad 2. These findings add to our understanding of how developmental and tumorigenic signals control Arf expression in vivo and in cultured MEFs.

Accelerated DNA replication in E2F1- and E2F2-deficient macrophages leads to induction of the DNA damage response and p21(CIP1)-dependent senescence

E2F1-3 proteins appear to have distinct roles in progenitor cells and in differentiating cells undergoing cell cycle exit. However, the function of these proteins in paradigms of terminal differentiation that involve continued cell division has not been examined. Using compound E2F1/E2F2-deficient mice, we have examined the effects of E2F1 and E2F2 loss on the differentiation and simultaneous proliferation of bone-marrow-derived cells toward the macrophage lineage. We show that E2F1/E2F2 deficiency results in accelerated DNA replication and cellular division during the initial cell division cycles of bone-marrow-derived cells, arguing that E2F1/E2F2 are required to restrain proliferation of pro-monocyte progenitors during their differentiation into macrophages, without promoting their cell cycle exit. Accelerated proliferation is accompanied by early expression of DNA replication and cell cycle regulators. Remarkably, rapid proliferation of E2F1/E2F2 compound mutant cultures is temporally followed by induction of a DNA damage response and the implementation of a p21(CIP1)-dependent senescence. We further show that differentiating E2F1/E2F2-knockout macrophages do not trigger a DNA damage response pathway in the absence of DNA replication. These findings underscore the relevance of E2F1 and E2F2 as suppressors of hematopoietic progenitor expansion. Our data indicate that their absence in differentiating macrophages initiates a senescence program that results from enforcement of a DNA damage response triggered by DNA hyper-replication.

Tandem E2F binding sites in the promoter of the p107 cell cycle regulator control p107 expression and its cellular functions

The retinoblastoma tumor suppressor (Rb) is a potent and ubiquitously expressed cell cycle regulator, but patients with a germline Rb mutation develop a very specific tumor spectrum. This surprising observation raises the possibility that mechanisms that compensate for loss of Rb function are present or activated in many cell types. In particular, p107, a protein related to Rb, has been shown to functionally overlap for loss of Rb in several cellular contexts. To investigate the mechanisms underlying this functional redundancy between Rb and p107 in vivo, we used gene targeting in embryonic stem cells to engineer point mutations in two consensus E2F binding sites in the endogenous p107 promoter. Analysis of normal and mutant cells by gene expression and chromatin immunoprecipitation assays showed that members of the Rb and E2F families directly bound these two sites. Furthermore, we found that these two E2F sites controlled both the repression of p107 in quiescent cells and also its activation in cycling cells, as well as in Rb mutant cells. Cell cycle assays further indicated that activation of p107 transcription during S phase through the two E2F binding sites was critical for controlled cell cycle progression, uncovering a specific role for p107 to slow proliferation in mammalian cells. Direct transcriptional repression of p107 by Rb and E2F family members provides a molecular mechanism for a critical negative feedback loop during cell cycle progression and tumorigenesis. These experiments also suggest novel therapeutic strategies to increase the p107 levels in tumor cells.

The retinoblastoma tumor suppressor Rb belongs to a family of cell cycle inhibitors along with the related proteins p107 and p130. Strong evidence indicates that the three family members have both specific and overlapping functions and expression patterns in mammalian cells, including in cancer cells. However, the molecular mechanisms underlying the functional differences and similarities among Rb, p107, and p130 are still poorly understood. One proposed mechanism of compensation is a negative feedback loop involving increased p107 transcription in Rb-deficient cells. To dissect the mechanisms controlling p107 expression in both wild-type and Rb-deficient cells, we have engineered inactivating point mutations into the E2F binding sites in the endogenous p107 promoter using gene targeting in mouse embryonic stem cells. Gene expression and DNA binding assays revealed that these two sites are essential for the control of p107 transcription in wild-type and Rb mutant cells, and cell cycle assays showed their importance for normal functions of p107. These experiments identify a key node in cell cycle regulatory networks.

Epigenetic regulation of p16Ink4a and Arf by JDP2 in cellular senescence

In response to accumulating cellular stress, cells protect themselves from abnormal growth by entering the senescent stage. Senescence is controlled mainly by gene products from the p16Ink4a/Arf locus. In mouse cells, the expression of p16Ink4a and Arf increases continuously during proliferation in cell culture. Transcription from the locus is under complex control. p16Ink4a and Arf respond independently to positive and negative signals, and the entire locus is epigenetically suppressed by histone methylation that depends on the Polycomb repressive complex-1 and -2 (PRC1 and PRC2). In fact, the PRCs associate with the p16Ink4a/Arf locus in young proliferating cells and dissociate in aged senescent cells. Thus, it seems that chromatin-remodeling factors that regulate association and dissociation of PRCs might be important players in the senescence program. Here, we summarize the molecular mechanisms that mediate cellular aging and introduce the Jun dimerization protein 2 (JDP2) as a factor that regulates replicative senescence by mediating dissociation of PRCs from the p16Ink4a/Arf locus.

Division and apoptosis of E2f-deficient retinal progenitors

The activating E2f transcription factors (E2f1, E2f2 and E2f3) induce transcription and are widely viewed as essential positive cell cycle regulators. Indeed, they drive cells out of quiescence, and the 'cancer cell cycle' in Rb1 null cells is E2f-dependent. Absence of activating E2fs in flies or mammalian fibroblasts causes cell cycle arrest, but this block is alleviated by removing repressive E2f or the tumour suppressor p53, respectively. Thus, whether activating E2fs are indispensable for normal division is an area of debate. Activating E2fs are also well known pro-apoptotic factors, providing a defence against oncogenesis, yet E2f1 can limit irradiation-induced apoptosis. In flies this occurs through repression of hid (also called Wrinkled; Smac/Diablo in mammals). However, in mammals the mechanism is unclear because Smac/Diablo is induced, not repressed, by E2f1, and in keratinocytes survival is promoted indirectly through induction of DNA repair targets. Thus, a direct pro-survival function for E2f1-3 and/or its relevance beyond irradiation has not been established. To address E2f1-3 function in normal cells in vivo we focused on the mouse retina, which is a relatively simple central nervous system component that can be manipulated genetically without compromising viability and has provided considerable insight into development and cancer. Here we show that unlike fibroblasts, E2f1-3 null retinal progenitor cells or activated Müller glia can divide. We attribute this effect to functional interchangeability with Mycn. However, loss of activating E2fs caused downregulation of the p53 deacetylase Sirt1, p53 hyperacetylation and elevated apoptosis, establishing a novel E2f-Sirt1-p53 survival axis in vivo. Thus, activating E2fs are not universally required for normal mammalian cell division, but have an unexpected pro-survival role in development.

Emerging roles of E2Fs in cancer: an exit from cell cycle control

Mutations of the retinoblastoma tumour suppressor gene (RB1) or components regulating the RB pathway have been identified in almost every human malignancy. The E2F transcription factors function in cell cycle control and are intimately regulated by RB. Studies of model organisms have revealed conserved functions for E2Fs during development, suggesting that the cancer-related proliferative roles of E2F family members represent a recent evolutionary adaptation. However, given that some human tumours have concurrent RB1 inactivation and E2F amplification and overexpression, we propose that there are alternative tumour-promoting activities for the E2F family, which are independent of cell cycle regulation.

p53--a Jack of all trades but master of none

Cancers are rare because their evolution is actively restrained by a range of tumour suppressors. Of these p53 seems unusually crucial as either it or its attendant upstream or downstream pathways are inactivated in virtually all cancers. p53 is an evolutionarily ancient coordinator of metazoan stress responses. Its role in tumour suppression is likely to be a relatively recent adaptation, which is only necessary when large, long-lived organisms acquired the sufficient size and somatic regenerative capacity to necessitate specific mechanisms to reign in rogue proliferating cells. However, such evolutionary reappropriation of this venerable transcription factor entails compromises that restrict its efficacy as a tumour suppressor.

Retinoblastoma protein plays multiple essential roles in the terminal differentiation of Sertoli cells

Retinoblastoma protein (RB) plays crucial roles in cell cycle control and cellular differentiation. Specifically, RB impairs the G(1) to S phase transition by acting as a repressor of the E2F family of transcriptional activators while also contributing towards terminal differentiation by modulating the activity of tissue-specific transcription factors. To examine the role of RB in Sertoli cells, the androgen-dependent somatic support cell of the testis, we created a Sertoli cell-specific conditional knockout of Rb. Initially, loss of RB has no gross effect on Sertoli cell function because the mice are fertile with normal testis weights at 6 wk of age. However, by 10-14 wk of age, mutant mice demonstrate severe Sertoli cell dysfunction and infertility. We show that mutant mature Sertoli cells continue cycling with defective regulation of multiple E2F1- and androgen-regulated genes and concurrent activation of apoptotic and p53-regulated genes. The most striking defects in mature Sertoli cell function are increased permeability of the blood-testis barrier, impaired tissue remodeling, and defective germ cell-Sertoli cell interactions. Our results demonstrate that RB is essential for proper terminal differentiation of Sertoli cells.

Expression of the Arf tumor suppressor gene is controlled by Tgfbeta2 during development

The Arf tumor suppressor (also known as Cdkn2a) acts as an oncogene sensor induced by ;abnormal' mitogenic signals in incipient cancer cells. It also plays a crucial role in embryonic development: newborn mice lacking Arf are blind due to a pathological process resembling severe persistent hyperplastic primary vitreous (PHPV), a human eye disease. The cell-intrinsic mechanism implied in the oncogene sensor model seems unlikely to explain Arf regulation during embryo development. Instead, transforming growth factor beta2 (Tgfbeta2) might control Arf expression, as we show that mice lacking Tgfbeta2 have primary vitreous hyperplasia similar to Arf(-/-) mice. Consistent with a potential linear pathway, Tgfbeta2 induces Arf transcription and p19(Arf) expression in cultured mouse embryo fibroblasts (MEFs); and Tgfbeta2-dependent cell cycle arrest in MEFs is maintained in an Arf-dependent manner. Using a new model in which Arf expression can be tracked by beta-galactosidase activity in Arf(lacZ/+) mice, we show that Tgfbeta2 is required for Arf transcription in the developing vitreous as well as in the cornea and the umbilical arteries, two previously unrecognized sites of Arf expression. Chemical and genetic strategies show that Arf promoter induction depends on Tgfbeta receptor activation of Smad proteins; the induction correlates with Smad2 phosphorylation in MEFs and Arf-expressing cells in vivo. Chromatin immunoprecipitation shows that Smads bind to genomic DNA proximal to Arf exon 1beta. In summary, Tgfbeta2 and p19(Arf) act in a linear pathway during embryonic development. We present the first evidence that p19(Arf) expression can be coupled to extracellular cues in normal cells and suggest a new mechanism for Arf control in tumor cells.

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.

Escape from p53-mediated tumor surveillance in neuroblastoma: switching off the p14(ARF)-MDM2-p53 axis

A primary failsafe program against unrestrained proliferation and oncogenesis is provided by the p53 tumor suppressor protein, inactivation of which is considered as a hallmark of cancer. Intriguingly, mutations of the TP53 gene are rarely encountered in neuroblastoma tumors, suggesting that alternative p53-inactivating lesions account for escape from p53 control in this childhood malignancy. Several recent studies have shed light on the mechanisms by which neuroblastoma cells circumvent the p53-driven antitumor barrier. We review here these mechanisms for evasion of p53-mediated growth control and conclude that deregulation of the p14(ARF)-MDM2-p53 axis seems to be the principal mode of p53 inactivation in neuroblastoma, opening new perspectives for targeted therapeutic intervention.

Sin3B expression is required for cellular senescence and is up-regulated upon oncogenic stress

Serial passage of primary mammalian cells or strong mitogenic signals induce a permanent exit from the cell cycle called senescence. A characteristic of senescent cells is the heterochromatinization of loci encoding pro-proliferative genes, leading to their transcriptional silencing. Senescence is thought to represent a defense mechanism against uncontrolled proliferation and cancer. Consequently, genetic alterations that allow senescence bypass are associated with susceptibility to oncogenic transformation. We show that fibroblasts genetically inactivated for the chromatin-associated Sin3B protein are refractory to replicative and oncogene-induced senescence. Conversely, overexpression of Sin3B triggers senescence and the formation of senescence-associated heterochromatic foci. Although Sin3B is strongly up-regulated upon oncogenic stress, decrease in expression of Sin3B is associated with tumor progression in vivo, suggesting that expression of Sin3B may represent a barrier against transformation. Together, these results underscore the contribution of senescence in tumor suppression and suggest that expression of chromatin modifiers is modulated at specific stages of cellular transformation. Consequently, these findings suggest that modulation of Sin3B-associated activities may represent new therapeutic opportunities for treatment of cancers.

ARF stimulates XPC to trigger nucleotide excision repair by regulating the repressor complex of E2F4

The tumour suppressor ARF (alternative reading frame), which is mutated or silenced in various tumours, has a crucial role in tumour surveillance to suppress unwarranted cell growth and proliferation. ARF has also been linked to the DNA-damage-induced response of p53 because of its ability to inhibit murine double minute 2 (MDM2). Here, however, we provide genetic evidence for a role of ARF in nucleotide excision repair (NER) that is independent of p53. Cells lacking ARF are deficient in NER. Expression of ARF restores the repair activity, which coincides with increased expression of the damaged-DNA recognition protein xeroderma pigmentosum, complementation group C (XPC). We provide evidence that, by disrupting the interaction between E2F transcription factor 4 (E2F4) and DRTF polypeptide 1 (DP1), ARF reduces the interaction of the E2F4-p130 repressor complex with the promoter of XPC to ensure high-level expression of XPC. Together, our results point to an important 'care-taker'-type tumour-suppression function for ARF in NER through the increased expression of XPC.

Cellular senescence: hot or what?

The phenomenon of replicative senescence was first observed more than 40 years ago by Hayflick who noted the inability of cultured human fibroblasts to proliferate indefinitely. The recent discovery that cellular senescence is triggered by many different activated oncogenes has led to the notion that senescence, like oncogene-induced apoptosis, serves as a critical and cell-autonomous tumor preventive mechanism. Both the DNA damage response and the ARF tumor suppressor have been mechanistically implicated in oncogene-induced senescence and the relative contributions of, and potential interactions between, these two pathways remain subjects of a lively debate. More recently, the discovery that cellular senescence can be bypassed during the epithelial-mesenchymal transition (EMT) that typically accompanies tumor progression, the observation that organ fibrosis is controlled by cellular senescence and, most noticeably, the mounting evidence linking cellular senescence to inflammation, make cellular senescence a still flaming hot subject after all these years.

E2f3b plays an essential role in myogenic differentiation through isoform-specific gene regulation

Current models posit that E2F transcription factors can be divided into members that either activate or repress transcription, in part through collaboration with the retinoblastoma (pRb) tumor suppressor family. The E2f3 locus encodes E2f3a and E2f3b proteins, and available data suggest that they regulate cell cycle-dependent gene expression through opposing transcriptional activating and repressing activities in growing and quiescent cells, respectively. However, the role, if any, of E2F proteins, and in particular E2f3, in myogenic differentiation is not well understood. Here, we dissect the contributions of E2f3 isoforms and other activating and repressing E2Fs to cell cycle exit and differentiation by performing genome-wide identification of isoform-specific targets. We show that E2f3a and E2f3b target genes are involved in cell growth, lipid metabolism, and differentiation in an isoform-specific manner. Remarkably, using gene silencing, we show that E2f3b, but not E2f3a or other E2F family members, is required for myogenic differentiation, and that this requirement for E2f3b does not depend on pRb. Our functional studies indicate that E2f3b specifically attenuates expression of genes required to promote differentiation. These data suggest how diverse E2F isoforms encoded by a single locus can play opposing roles in cell cycle exit and differentiation.

E2f6 and Bmi1 cooperate in axial skeletal development

Bmi1 is a Polycomb Group protein that functions as a component of Polycomb Repressive Complex 1 (PRC1) to control axial skeleton development through Hox gene repression. Bmi1 also represses transcription of the Ink4a-Arf locus and is consequently required to maintain the proliferative and self-renewal properties of hematopoietic and neural stem cells. Previously, one E2F family member, E2F6, has been shown to interact with Bmi1 and other known PRC1 components. However, the biological relevance of this interaction is unknown. In this study, we use mouse models to investigate the interplay between E2F6 and Bmi1. This analysis shows that E2f6 and Bmi1 cooperate in the regulation of Hox genes, and consequently axial skeleton development, but not in the repression of the Ink4a-Arf locus. These findings underscore the significance of the E2F6-Bmi1 interaction in vivo and suggest that the Hox and Ink4a-Arf loci are regulated by somewhat different mechanisms.

E2F - at the crossroads of life and death

The retinoblastoma tumor suppressor, pRb, restricts cell-cycle progression mainly by regulating members of the E2F-transcription-factor family. The Rb pathway is often inactivated in human tumors, resulting in deregulated-E2F activity that promotes proliferation or cell death, depending on the cellular context. Specifically, the outcome of deregulated-E2F activity is determined by integration of signals coming from the cellular DNA and the external environment. Alterations in cell proliferation and cell-death pathways are key features of transformed cells and, therefore, an understanding of the variables that determine the outcome of E2F activation is pivotal for cancer research and treatment. In this review, we discuss recent studies that have elucidated some of the signals affecting E2F activity and that have revealed additional E2F targets and functions, thereby enriching the understanding of this versatile transcription-factor family.

E2f3a and E2f3b make overlapping but different contributions to total E2f3 activity

The E2f transcription factors are key downstream targets of the retinoblastoma protein tumor suppressor that control cell proliferation. E2F3 has garnered particular attention because it is amplified in various human tumors. E2f3 mutant mice typically die around birth and E2f3-deficient cells have a proliferation defect that correlates with impaired E2f target gene activation and also induction of p19(Arf) and p53. The E2f3 locus encodes two isoforms, E2f3a and E2f3b, which differ in their N-termini. However, it is unclear how E2f3a versus E2f3b contributes to E2f3's requirement in either proliferation or development. To address this, we use E2f3a- and E2f3b-specific knockouts. We show that inactivation of E2f3a results in a low penetrance proliferation defect in vitro whereas loss of E2f3b has no effect. This proliferation defect appears insufficient to disrupt normal development as E2f3a and E2f3b mutant mice are both fully viable and have no detectable defects. However, when combined with E2f1 mutation, inactivation of E2f3a, but not E2f3b, causes significant proliferation defects in vitro, neonatal lethality and also a striking cartilage defect. Thus, we conclude that E2f3a and E2f3b have largely overlapping functions in vivo and that E2f3a can fully substitute for E2f1 and E2f3 in most murine tissues.

Hypoxia response and microRNAs: no longer two separate worlds

MicroRNAs (miRs) are short non-coding transcripts involved in a wide variety of cellular processes. Several recent studies have established a link between hypoxia, a well-documented component of the tumour microenvironment, and specific miRs. One member of this class, miR-210, was identified as hypoxia inducible in all the cell types tested, and is overexpressed in most cancer types. Its hypoxic induction is dependent on a functional hypoxia-inducible factor (HIF), thus extending the transcriptional repertoire of the latter beyond ‘classic’ genes. From a clinical standpoint, miR-210 overexpression has been associated with adverse prognosis in breast tumours and been detected in serum of lymphoma patients and could serve as a tool to define hypoxic malignancies. We discuss the role of miR-210 and its emerging targets, as well as possible future directions for clinical applications in oncology and ischaemic disorders.

Small tumor antigen of polyomaviruses: role in viral life cycle and cell transformation

The regulatory proteins of polyomaviruses, including small and large T antigens, play important roles, not only in the viral life cycle but also in virus-induced cell transformation. Unlike many other tumor viruses, the transforming proteins of polyomaviruses have no cellular homologs but rather exert their effects mostly by interacting with cellular proteins that control fundamental processes in the regulation of cell proliferation and the cell cycle. Thus, they have proven to be valuable tools to identify specific signaling pathways involved in tumor progression. Elucidation of these pathways using polyomavirus transforming proteins as tools is critically important in understanding fundamental regulatory mechanisms and hence to develop effective therapeutic strategies against cancer. In this short review, we will focus on the structural and functional features of one polyomavirus transforming protein, that is, the small t-antigen of the human neurotropic JC virus (JCV) and the simian virus, SV40.

Signal transduction involving the dmp1 transcription factor and its alteration in human cancer

Dmp1 (cyclin D-interacting myb-like protein 1; also called Dmtf1) is a transcription factor that has been isolated in a yeast two-hybrid screen through its binding property to cyclin D2. Dmp1 directly binds to and activates the Arf promoter and induces Arf-p53-dependent cell cycle arrest in primary cells. D-type cyclins usually inhibit Dmp1-mediated transcription in a Cdk-independent fashion; however, Dmp1 shows synergistic effects with D-cyclins on the Arf promoter. Ras or Myc oncogene-induced tumor formation is accelerated in both Dmp1^+/− and Dmp1^−/− mice with no significant differences between Dmp1^+/− and Dmp1^−/−. Thus, Dmp1 is haplo-insufficient for tumor suppression. Tumors from Dmp1^−/− or Dmp1^+/− mice often retain wild-type Arf and p53, suggesting that Dmp1 is a physiological regulator of the Arf-p53 pathway. The Dmp1 promoter is activated by oncogenic Ras-Raf signaling, while it is repressed by physiological mitogenic stimuli, overexpression of E2F proteins, and genotoxic stimuli mediated by NF-κB. The human DMP1 gene (hDMP1) is located on chromosome 7q21 and is hemizygously deleted in approximately 40% of human lung cancers, especially those that retain normal INK4a/ARF and P53 loci. Thus, hDMP1 is clearly involved in human carcinogenesis, and tumors with hDMP1 deletion may constitute a discrete disease entity.

Polycomb complexes regulate cellular senescence by repression of ARF in cooperation with E2F3

Cellular senescence is a program in normal cells triggered in response to various types of stress that cells experience when they are explanted into culture. In this study, functional analyses on the role of the class II polycomb complex in cellular senescence were performed using mouse embryo fibroblasts (MEFs) with a genetically deleted member of the complex, Mel18. Mel18-null MEFs undergo typical premature senescence accompanied by the up-regulation of ARF/p53/p16(INK4a) and decrease of Ring1b/Bmi1. Our results demonstrated that ARF or p53 deletion cancels the senescence in Mel18-null MEFs, and the fact that p16(INK4a) is up-regulated in double-null MEFs suggests that the ARF/p53 pathway plays a central role in stress-induced senescence. The in vivo binding of Ring1b and E2F3b to the ARF promoter decreased progressively in senescence, and Mel18 inactivation accelerated the exfoliation of Ring1b/E2F3b from the promoter sequence, indicating the cooperation of polycombs/E2F3b on ARF expression and cellular senescence. Taken together, it seems that class II polycomb proteins and E2F3b dually control cellular senescence via the ARF/p53 pathway.

TGFbeta-mediated formation of pRb-E2F complexes in human myeloid leukemia cells

TGFbeta is well known for its inhibitory effect on cell cycle G1 checkpoint kinases. However, its role in the control of pRb-E2F complexes is not well established. TGFbeta inhibits phosphorylation of pRb at several serine and threonine residues and regulates the association of E2F transcription factors with pRb family proteins. Recent studies found that predominantly E2F-4, p130, and histone deacetylase (HDAC) are found to bind to corresponding E2F-responsive promoters in G0/G1 phase. As cells progress through mid-G1, p130-E2F4 complex are replaced by p107-E2F4 followed by activators E2F1, 2, and 3. pRb was not detectable in the promoters containing the E2F-responsive site in cycling cells but was associated with E2F4-p130 complexes or E2F4-p107 complexes during G0/G1 phase. In human myeloid leukemia cell line, MV4-11, TGFbeta upregulated pRb-E2F-4 and p130-E2F-4, and downregulated p107-E2F-4 complexes. However, pRB-E2F1 and pRb-E2F3 complexes were found in proliferating cells but not in TGFbeta arrested G1 cells. In addition, electrophoretic gel mobility shift assay (EMSA) could not detect pRb-E2F DNA-binding activities either in S or G1 phase but exhibited the existence of p107-E2F4 in proliferating cells and p130-E2F4 complexes in TGFbeta-arrested G1 cells, respectively. Our data suggest that p107 and p130, but not pRb, and the repressor E2F, but not activator E2Fs, play a critical role in regulating E2F-responsive gene expression in TGFbeta-mediated cell cycle control in human myeloid leukemia cells.

Gene Ontology-driven transcriptional analysis of CD34+ cell-initiated megakaryocytic cultures identifies new transcriptional regulators of megakaryopoiesis

Differentiation of hematopoietic stem and progenitor cells is an intricate process controlled in large part at the level of transcription. While some key megakaryocytic transcription factors have been identified, the complete network of megakaryocytic transcriptional control is poorly understood. Using global gene expression microarray analysis, Gene Ontology-based functional annotations, and a novel interlineage comparison with parallel, isogenic granulocytic cultures as a negative control, we closely examined the mRNA level of transcriptional regulators in megakaryocytes derived from human mobilized peripheral blood CD34(+) hematopoietic cells. This approach identified 199 differentially expressed transcription factors or transcriptional regulators. We identified and detailed the transcriptional kinetics of most known megakaryocytic transcription factors including GATA1, FLI1, and MAFG. Furthermore, many genes with transcription factor activity or transcription factor binding activity were identified in megakaryocytes that had not previously been associated with that lineage, including BTEB1, NR4A2, FOXO1A, MEF2C, HDAC5, VDR, and several genes associated with the tumor suppressor p53 (HIPK2, FHL2, and TADA3L). Protein expression and nuclear localization were confirmed in megakaryocytic cells for four of the novel candidate megakaryocytic transcription factors: FHL2, MXD1, E2F3, and RFX5. In light of the hypothesis that transcription factors expressed in a particular differentiation program are important contributors to such a program, these data substantially expand our understanding of transcriptional regulation in megakaryocytic differentiation of stem and progenitor cells.

Cellular signaling in normal and cancerous stem cells

Self-renewing divisions of normal and cancerous stem cells are responsible for the initiation and maintenance of normal and certain cancerous tissues, respectively. Recent findings suggest that tumor surveillance mechanisms can reduce regenerative capacity and frequency of normal stem cells, thereby contributing to tissue aging. Signaling pathways promoting self-renewal of stem cells can also drive proliferation in cancer. The BMI-1 proto-oncogene is required for the maintenance of tissue-specific stem cells and is involved in carcinogenesis within the same tissues. BMI-1 promotes self-renewal of stem cells largely by interfering with two central cellular tumor suppressor pathways, p16(Ink4a)/retinoblastoma protein (Rb) and ARF/p53, whose disruption is a hallmark of cancer. Nucleolin, an Rb-associated protein, is abundant in proliferating cancerous cells and likely contributes to the maintenance of human CD34-positive stem/progenitor cells of hematopoiesis. Elucidation of the involvement of proto-oncogenes and tumor suppressors in the maintenance of stem cells might have therapeutic implications.

Versatile functions of p53 protein in multicellular organisms

The p53 tumor suppressor plays a pivotal role in multicellular organism by enforcing benefits of the organism over those of an individual cell. The task of p53 is to control the integrity and correctness of all processes in each individual cell and in the organism as a whole. Information about the state of ongoing events in the cell is gathered through multiple signaling pathways that convey signals modifying activities of p53. Changes in the activities depend on the character of damages or deviations from optimum in processes, and the activity of p53 changes depending on the degree of the aberration, which results in either stimulation of repair processes and protective mechanisms, or the cessation of further cell divisions and the induction of programmed cell death. The strategy of p53 ensures genetic identity of cells and prevents the selection of abnormal cells. By accomplishing these strategic tasks, p53 may use a wide spectrum of activities, such as its ability to function as a transcription factor, by inducing or repressing different genes, or as an enzyme, by acting as an exonuclease during DNA reparation, or as an adaptor or a regulatory protein, intervening into functions of numerous signaling pathways. Loss of function of the p53 gene occurs in virtually every case of cancer, and deficiency in p53 is an unavoidable prerequisite to the development of malignancies. The functions of p53 play substantial roles in many other pathologies as well as in the aging process. This review is focused on strategies of the p53 gene, demonstrating individual mechanisms underlying its functions.

Inactivation of the Rb pathway and overexpression of both isoforms of E2F3 are obligate events in bladder tumours with 6p22 amplification

E2F3 and CDKAL1 are candidate genes from the 6p22 region frequently amplified in bladder cancer. Expression of E2F3 isoforms (E2F3a and b) and CDKAL1 were examined and modulated in 6p22-amplified bladder cell lines. Eight lines with amplification showed overexpression of both E2F3 isoforms and CDKAL1. shRNA-mediated knockdown of CDKAL1 had no effect on proliferation. Knockdown of E2F3a or E2F3b alone induced antiproliferative effects, with the most significant effect on proliferation being observed when both isoforms were knocked down together. As E2Fs interact with the Rb tumour suppressor protein, Rb expression was analysed. There was a striking relationship between 6p22.3 amplification, E2F3 overexpression and lack of Rb expression. This was also examined in primary bladder tumours. Array-CGH detected 6p22.3 amplification in 8/91 invasive tumours. Five were studied in more detail. Four showed 13q14.2 loss (including RB1) and expressed no Rb protein. In the fifth, 13q was unaltered but the CDKN2A locus was deleted. This tumour was negative for p16 and positive for Rb protein. As p16 is a negative regulator of the Rb pathway, its loss represents an alternative mechanism for inactivation. Indeed, a phospho-specific Rb antibody showed much Rb protein in a hyperphosphorylated (inactive) form. We conclude that inactivation of the Rb pathway is required in addition to E2F3 overexpression in this subset of bladder tumours.

Life and death decisions by the E2F transcription factors

The E2F transcription factors are critical regulators of genes required for appropriate progression through the cell cycle, and in special circumstances they can also promote the expression of another class of genes that function in the apoptotic program. Since E2Fs can initiate both cell proliferation and cell death, it is not surprising that the pro-apoptotic capacity of these proteins is subject to complex regulation. Recent study has expanded our knowledge of the factors influencing E2F-induced apoptosis as well as downstream targets of E2F in this process.

Novel and unexpected role for the tumor suppressor ARF in viral infection surveillance

Virus infection induces the synthesis of interferons which, in turn, stimulate the expression of hundreds of cellular genes, any of those denominated viral-stress-inducible genes. Among interferon-upregulated genes, also triggered by oncogenic viruses, several tumor-suppressor genes can also be listed. A correlation between the tumor suppressor alternative reading frame (ARF) and virus replication was noted some time ago. Yang and colleagues in 2001 demonstrated that p14ARF modulated the cytolytic effect of the E1B-deleted adenovirus ONYX-015 in mesothelioma cells with wild-type p53, and expression of p14ARF attenuated the cytolytic effect of the virus. Later, in 2006, Garcia and colleagues identified ARF as a gene product with a role in reducing the sensitivity of cells to infection by several viruses, showing an inverse relationship between doses of ARF and levels of virus replication. Additionally, the same authors presented a number of experiments designed to illustrate the molecular mechanisms underlying the decrease of virus replication upon ARF overexpression, demonstrating a p53-independent ARF function. ARF is the latest tumor suppressor added to the list of the cellular genes upregulated by type I interferon that possesses antiviral activity. The antiviral role of other tumor suppressor pathways targeted by both interferons and oncogenic viruses requires further investigation.

The Myc-evoked DNA damage response accounts for treatment resistance in primary lymphomas in vivo

In addition to the ARF/p53 pathway, the DNA damage response (DDR) has been recognized as another oncogene-provoked anticancer barrier in early human tumorigenesis leading to apoptosis or cellular senescence. DDR mutations may promote tumor formation, but their impact on treatment outcome remains unclear. In this study, we generated ataxia telangiectasia mutated (Atm)–proficient and -deficient B-cell lymphomas in Eμ-myc transgenic mice to examine the role of DDR defects in lymphomagenesis and treatment sensitivity. Atm inactivation accelerated development of lymphomas, and their DNA damage checkpoint defects were virtually indistinguishable from those observed in Atm+/+-derived lymphomas that spontaneously inactivated the proapoptotic Atm/p53 cascade in response to Myc-evoked reactive oxygen species (ROS). Importantly, acquisition of DDR defects, but not selection against the ARF pathway, could be prevented by lifelong exposure to the ROS scavenger N-acetylcysteine (NAC) in vivo. Following anticancer therapy, DDR-compromised lymphomas displayed apoptotic but, surprisingly, no senescence defects and achieved a much poorer long-term outcome when compared with DDR-competent lymphomas treated in vivo. Hence, Atm eliminates preneoplastic lesions by converting oncogenic signaling into apoptosis, and selection against an Atm-dependent response promotes formation of lymphomas with predetermined treatment insensitivity.

Deregulated E2f-2 underlies cell cycle and maturation defects in retinoblastoma null erythroblasts

By assessing the contribution of deregulated E2F activity to erythroid defects in Rb null mice, we have identified E2f-2 as being upregulated in end-stage red cells, where we show it is the major pRb-associated E2f and the predominant E2f detected at key target gene promoters. Consistent with its expression pattern, E2f-2 loss restored terminal erythroid maturation to Rb null red cells, including the ability to undergo enucleation. Deletion of E2f-2 also extended the life span of Rb null mice despite persistent defects in placental development, indicating that deregulated E2f-2 activity in differentiating erythroblasts contributes to the premature lethality of Rb null mice. We show that the aberrant entry of Rb null erythroblasts into S phase at times in differentiation when wild-type erythroblasts are exiting the cell cycle is inhibited by E2f-2 deletion. E2f-2 loss induced cell cycle arrest in both wild-type and Rb null erythroblasts and was associated with increased DNA double-strand breaks. These results implicate deregulated E2f-2 in the cell cycle defects observed in Rb null erythroblasts and reveal a novel role for E2f-2 during terminal red blood cell differentiation. The identification of a tissue-restricted role for E2f-2 in erythropoiesis highlights the nonredundant nature of E2f transcription factor activities in cell growth and differentiation.

Patterns of EGFR, HER2, TP53, and KRAS mutations of p14arf expression in non-small cell lung cancers in relation to smoking history

Mutations in the tyrosine kinase domain of the epidermal growth factor receptor EGFR are common in non-small cell lung cancer (NSCLC) of never smokers, whereas HER2 mutations are rare. We have analyzed EGFR and HER2 mutations and the expression of the two products of the CDKN2A gene (p14(arf) and p16(INK4a)) in 116 NSCLC that have been previously analyzed for TP53 and KRAS mutations in relation to smoking history of patients. EGFR mutations were detected in 20 of 116 (17%) tumors, whereas five (4.3%) tumors contained HER2 mutations. No tumor contained both mutations. Of tumors with EGFR or HER2 mutation, 72% were adenocarcinomas, 68% were from never smokers, and 32% were from former smokers. EGFR but not HER2 mutations were mutually exclusive with KRAS mutation. Among never smokers, 11 of 16 tumors with EGFR mutation also had TP53 mutation, in contrast with two of 17 tumors without EGFR mutation (P = 0.0008). Expression of p14(arf), but not p16(ink4a), was more frequently down-regulated in never smokers (62.5%) than ever smokers (35%; P = 0.008). All tumors with EGFR or HER2 mutations and wild-type TP53 showed down-regulation of p14(arf) expression. These observations suggest that functional inactivation of the p14(arf)/p53 connection is required in tumors with EGFR or HER2 mutations, consistent with the notion that these proteins are part of a fail-safe mechanism protecting cells against untimely or excessive mitotic signals.

Comparative analysis of E2F family member oncogenic activity

The E2F family of transcription factors consists of nine members with both distinct and overlapping functions. These factors are situated downstream of growth factor signaling cascades, where they play a central role in cell growth and proliferation through their ability to regulate genes involved in cell cycle progression. For this reason, it is likely that the members of the E2F family play a critical role during oncogenesis. Consistent with this idea is the observation that some tumors exhibit deregulated expression of E2F proteins. In order to systematically compare the oncogenic capacity of these family members, we stably over-expressed E2F1 through 6 in non-transformed 3T3 fibroblasts and assessed the ability of these transgenic cell lines to grow under conditions of low serum, as well as to form colonies in soft agar. Our results show that these six E2F family members can be divided into three groups that exhibit differential oncogenic capacity. The first group consists of E2F2 and E2F3a, both of which have strong oncogenic capacity. The second group consists of E2F1 and E2F6, which were neutral in our assays when compared to control cells transduced with vector alone. The third group consists of E2F4 and E2F5, which generally act to repress E2F-responsive genes, and in our assays demonstrated a strong capacity to inhibit transformation. Our results imply that the pattern of expression of these six E2F family members in a cell could exert a strong influence over its susceptibility to oncogenic transformation.

Rb-mediated neuronal differentiation through cell-cycle-independent regulation of E2f3a

It has long been known that loss of the retinoblastoma protein (Rb) perturbs neural differentiation, but the underlying mechanism has never been solved. Rb absence impairs cell cycle exit and triggers death of some neurons, so differentiation defects may well be indirect. Indeed, we show that abnormalities in both differentiation and light-evoked electrophysiological responses in Rb-deficient retinal cells are rescued when ectopic division and apoptosis are blocked specifically by deleting E2f transcription factor (E2f) 1. However, comprehensive cell-type analysis of the rescued double-null retina exposed cell-cycle–independent differentiation defects specifically in starburst amacrine cells (SACs), cholinergic interneurons critical in direction selectivity and developmentally important rhythmic bursts. Typically, Rb is thought to block division by repressing E2fs, but to promote differentiation by potentiating tissue-specific factors. Remarkably, however, Rb promotes SAC differentiation by inhibiting E2f3 activity. Two E2f3 isoforms exist, and we find both in the developing retina, although intriguingly they show distinct subcellular distribution. E2f3b is thought to mediate Rb function in quiescent cells. However, in what is to our knowledge the first work to dissect E2f isoform function in vivo we show that Rb promotes SAC differentiation through E2f3a. These data reveal a mechanism through which Rb regulates neural differentiation directly, and, unexpectedly, it involves inhibition of E2f3a, not potentiation of tissue-specific factors.

The retinoblastoma protein (Rb), an important tumor suppressor, blocks division and death by inhibiting the E2f transcription factor family. In contrast, Rb is thought to promote differentiation by potentiating tissue-specific transcription factors, although differentiation defects in Rb null cells could be an indirect consequence of E2f-driven division and death. Here, we resolve different mechanisms by which Rb controls division, death, and differentiation in the retina. Removing E2f1 rescues aberrant division of differentiating Rb-deficient retinal neurons, as well as death in cells prone to apoptosis, and restores both normal differentiation and function of major cell types, such as photoreceptors. However, Rb-deficient starburst amacrine neurons differentiate abnormally even when E2f1 is removed, providing an unequivocal example of a direct role for Rb in neuronal differentiation. Rather than potentiating a cell-specific factor, Rb promotes starburst cell differentiation by inhibiting another E2f, E2f3a. This cell-cycle–independent activity broadens the importance of the Rb–E2f pathway, and suggests we should reassess its role in the differentiation of other cell types.

The retinoblastoma protein (Rb), a tumor suppressor, promotes the differentiation of starburst amacrine cells in the retina by inhibiting the transcription factor E2f3a, whereas it suppresses retinal cell division and death by inhibiting E2f1.

Proapoptotic role of novel gene-expression factors

The mechanisms that control cellular proliferation, as well as those related with programmed cell death or apoptosis, require precise regulation systems to prevent diseases such as cancer. Events related to cellular proliferation as well as those associated with apoptosis involve the regulation of gene expression carried out by three basic genetic expression regulation mechanisms: transcription, splicing of the primary transcript for mature mRNA formation, and RNA translation, a ribosomal machinery-dependent process for protein synthesis. While development of each one of these processes requires energy for recognition and assembly of a number of molecular complexes, it has been reported that an increased expression of several members of these protein complexes promotes apoptosis in distinct cell types. The question of how these factors interact with other proteins in order to incorporate themselves into the different transduction cascades and stimulate the development of programmed cell death, although nowadays actively studied, is still waiting for a clear-cut answer. This review focuses on the interactions established between different families of transcription, elongation, translation and splicing factors associated to the progression of apoptosis.

RNA polymerase III transcription is repressed in response to the tumour suppressor ARF

The tumour suppressor protein ARF provides a defence mechanism against hyperproliferative stresses that can result from the aberrant activation of oncogenes. Accordingly, ARF is silenced or deleted in many human cancers. Activation of ARF can arrest growth and cell cycle progression, or trigger apoptosis. A principle mediator of these effects is p53, which ARF stabilizes by binding and inhibiting MDM2. However, ARF has additional targets and remains able to block growth in the absence of p53, albeit less efficiently. For example, ARF can suppress rRNA production in a p53-independent manner. We have found that the synthesis of tRNA by RNA polymerase III is also inhibited in response to ARF. However, in contrast to its effects on rRNA synthesis, ARF is unable to inhibit tRNA gene transcription when p53 is ablated. These results add to the growing list of cellular changes that can be triggered by ARF induction.

E2F-1 regulates expression of FOXO1 and FOXO3a

E2F and FOXO transcription factors both play a role in neuronal apoptosis. In addition, both E2F-induced apoptosis and FOXO function are inhibited by the kinase Akt. We therefore tested whether FOXO is downstream of E2F-1 during neuronal apoptosis. We found that expression of endogenous FOXO1 and FOXO3a is induced by E2F-1. The presence of putative E2F binding sites in the promoters of both genes suggested that FOXO genes are direct targets of E2F-1. Indeed, a 4-hydroxytamoxifen activated E2F-1-ER fusion protein induced FOXO expression in the presence of cycloheximide. Moreover, E2F-1 activated the FOXO1 promoter in transient reporter assays, and E2F-1-ER as well as endogenous E2F bound to the FOXO1 promoter in vivo. Yet, E2F-1-mediated apoptosis of differentiated PC12 cells after withdrawal of NGF was not accompanied by changes in FOXO expression, indicating that no transcriptional induction of FOXO occurs during E2F-1-dependent neuronal apoptosis. In summary, our data identify E2F-1 as a first transcription factor regulating FOXO expression, providing a link between E2F and FOXO proteins in the control of cell fate.

MDMX: from bench to bedside

The tumor suppressor protein p53 is negatively regulated by Mdm2, a ubiquitin ligase protein that targets p53 for degradation. Mdmx (also known as Mdm4) is a relative of Mdm2 that was identified on the basis of its ability to physically interact with p53. An increasing body of evidence, including recent genetic studies, suggests that Mdmx also acts as a key negative regulator of p53. Aberrant expression of MDMX could thus contribute to tumor formation. Indeed, MDMX amplification and/or overexpression occurs in several diverse tumors. Strikingly, recent work identifies MDMX as a specific chemotherapeutic target for treatment of retinoblastoma. Specific MDMX antagonists should therefore be developed as a tool to ensure activation of `dormant' p53 activity in tumors that retain wild-type p53.

Dmp1 and tumor suppression

Dmp1 (cyclin D binding myb-like protein 1; also called Dmtf1) is a transcription factor that was isolated in a yeast two-hybrid screen through its binding property to cyclin D2. Although it was initially predicted to be involved in the cyclin D-Rb pathway, overexpression of Dmp1 in primary cells induces cell cycle arrest in an Arf, p53-dependent fashion. Dmp1 is a unique Arf regulator, the promoter of which is activated by oncogenic Ras-Raf signaling. Dmp1 expression is repressed by physiological mitogenic stimuli as well as by overexpressed E2F proteins; thus, it is a novel marker of cells that have exited from the cell cycle. Spontaneous and oncogene-induced tumor formation is accelerated in both Dmp1(+/-) and Dmp1(-/-) mice; the Dmp1(+/-) tumors often retain and express the wild-type allele; thus, Dmp1 is haplo-insufficient for tumor suppression. Tumors from Dmp1(+/-) and Dmp1(-/-) mice often retain wild-type Arf and p53, suggesting that Dmp1 is a physiological regulator of the Arf-p53 pathway. The human DMP1 (hDMP1) gene is located on chromosome 7q21, the locus of which is often deleted in myeloid leukemia and also in some types of solid tumors. Post-translational modification of Dmp1 and its role in human malignancy remain to be investigated.

E2f1, E2f2, and E2f3 control E2F target expression and cellular proliferation via a p53-dependent negative feedback loop

E2F-mediated control of gene expression is believed to have an essential role in the control of cellular proliferation. Using a conditional gene-targeting approach, we show that the targeted disruption of the entire E2F activator subclass composed of E2f1, E2f2, and E2f3 in mouse embryonic fibroblasts leads to the activation of p53 and the induction of p53 target genes, including p21(CIP1). Consequently, cyclin-dependent kinase activity and retinoblastoma (Rb) phosphorylation are dramatically inhibited, leading to Rb/E2F-mediated repression of E2F target gene expression and a severe block in cellular proliferation. Inactivation of p53 in E2f1-, E2f2-, and E2f3-deficient cells, either by spontaneous mutation or by conditional gene ablation, prevented the induction of p21(CIP1) and many other p53 target genes. As a result, cyclin-dependent kinase activity, Rb phosphorylation, and E2F target gene expression were restored to nearly normal levels, rendering cells responsive to normal growth signals. These findings suggest that a critical function of the E2F1, E2F2, and E2F3 activators is in the control of a p53-dependent axis that indirectly regulates E2F-mediated transcriptional repression and cellular proliferation.

Re-evaluating cell-cycle regulation by E2Fs

Activation of E2F transcription factors is thought to drive the expression of genes essential for the transition of cells from G1 to S phase and for the initiation of DNA replication. However, this textbook view of E2Fs is increasingly under challenge. Here we discuss an alternative model for how E2Fs may work.

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.

Proteomic analysis of the E2F1 response in p53-negative cancer cells: new aspects in the regulation of cell survival and death

E2F1 is an essential transcription factor that regulates cell-cycle progression and apoptosis. Overexpression of E2F1 sensitizes neoplastic cells to apoptosis and leads to tumor growth suppression, making it an interesting target for anticancer therapy. Use of E2F1 as a therapeutic, however, requires a detailed knowledge of the mechanisms by which it controls cellular proliferation and apoptosis, and of other potential E2F1 activities. In this study, a differential proteome analysis was performed to identify proteins associated with E2F1 activity in inducible p53-deficient Saos-2ERE2F1 osteosarcoma cells. 2-DE revealed a distinct protein profile at 32 h after E2F1 activation. Thirty-three proteins were reproducibly identified as either up-regulated or down-regulated. Proteins were identified by MALDI-MS. They included hitherto unknown E2F1 target proteins of cytoskeletal origin, chaperones, enzymes, proteasomal proteins, and several heterogeneous nuclear ribonucleoproteins, suggesting its role in the ER-stress response, protein degradation, and modulation of pre-mRNA splicing. Protein analysis-derived results were verified by Western blot using representative protein candidates. Thirteen identified proteins were the products of genes known to be cancer related. Thus, proteome analysis provides new information about the complexity of E2F1 activities in human cancer cells that may be considered when using E2F1 as a drug.

The STAT3 oncogene as a predictive marker of drug resistance

Constitutive activation of STAT3 (signal transducer and activator of transcription) has been reported in several primary cancers and tumor cell lines where it induces cell transformation through a combined inhibition of apoptosis and cell-cycle activation. Several studies have suggested that STAT3 prevents cell-cycle arrest and cell death through upregulation of survival proteins and downregulation of tumor suppressors. As a consequence of anti-apoptotic and proliferative lesions, we propose that this oncogenic pathway is also involved in intrinsic drug resistance and that STAT3-expressing tumors are resistant to chemotherapeutic agents. If this hypothesis is correct, the detection of the activated form of this protein should help to define subsets of tumors that fail to respond to chemotherapy. Furthermore, interfering with the STAT3 oncogenic pathway might restore the sensitivity to anticancer drugs.

Regulation of PAI-1 gene expression during adipogenesis

Obesity is characterized by elevated levels of circulating plasminogen activator inhibitor-1 (PAI-1), which contribute towards the development of secondary disorders such as type 2 diabetes mellitus and cardiovascular complications. This increase in plasma PAI-1 levels is attributed to an increase in PAI-1 derived from adipose tissue. This study shows that adipose tissue evolved into a major PAI-1 producing organ by gaining capacity during adipocyte differentiation to respond to inducers of PAI-1 transcription. This is mediated by a decrease in E2F1 protein levels, an increase in pRB levels and a decrease in pRB phosphorylation, all leading to a decrease in levels of free E2F, a known transcriptional repressor of PAI-1. Depletion of E2F1-3 was sufficient for inducers such as insulin to potently induce PAI-1 gene expression in pre-adipocytes. Conversely, forced release of pRB-bound endogenous E2F using cell-penetrating peptides can suppress PAI-1 gene expression in adipocytes. This study describes the novel paradigm of cellular differentiation-associated increase in PAI-1 gene expression which is mediated by a decrease in repressor activity, and describes a way of desensitising terminally differentiated cells to PAI-1 inducing agents by restoring endogenous repressor activity.

Inactivation of the p53 pathway in retinoblastoma

Most human tumours have genetic mutations in their Rb and p53 pathways, but retinoblastoma is thought to be an exception. Studies suggest that retinoblastomas, which initiate with mutations in the gene retinoblastoma 1 (RB1), bypass the p53 pathway because they arise from intrinsically death-resistant cells during retinal development. In contrast to this prevailing theory, here we show that the tumour surveillance pathway mediated by Arf, MDM2, MDMX and p53 is activated after loss of RB1 during retinogenesis. RB1-deficient retinoblasts undergo p53-mediated apoptosis and exit the cell cycle. Subsequently, amplification of the MDMX gene and increased expression of MDMX protein are strongly selected for during tumour progression as a mechanism to suppress the p53 response in RB1-deficient retinal cells. Our data provide evidence that the p53 pathway is inactivated in retinoblastoma and that this cancer does not originate from intrinsically death-resistant cells as previously thought. In addition, they support the idea that MDMX is a specific chemotherapeutic target for treating retinoblastoma.

The regulation of INK4/ARF in cancer and aging

Loss of the INK4a/ARF/INK4b locus on chromosome 9p21 is among the most frequent cytogenetic events in human cancer. The products of the locus--p15(INK4b), p16(INK4a), and ARF--play widespread and independent roles in tumor suppression. Recent data also suggest that expression of p16(INK4a) induces an age-dependent decrease in the proliferative capacity of certain tissue-specific stem cells and unipotent progenitors. Here, we discuss the regulation and role of p16(INK4a), ARF, and p15(INK4b) in cancer and aging.