Cloning, chromosomal location, and characterization of mouse E2F1

Molecular cloning and expression pattern of the DP members of the chicken E2F transcription factor

The DP proteins are components of the E2F transcription factor. They form heterodimers with the E2F proteins and these complexes bind efficiently to E2F response elements in promoters of genes that are involved in cell cycle regulation. The properties of the DP proteins are less documented than those of their E2F counterpart and the present work was aimed at characterizing avian DP genes (named chDP) and their products. Here we describe the cloning of the chicken homologues of the mammalian DP-1 and DP-2 proteins. This work also suggests that DP-2 isoforms have an additional 60 amino acid extension at the N-terminus compared to its human counterpart. Gel-shift assays and coimmunoprecipitation show that both DP-1 and DP-2 dimerize to chE2F-1 and activate transcription efficiently, as demonstrated by transient expression assays. However, contrary to the expression patterns exhibited by E2F-1 during the cell cycle or during neuroretina development, DP member's expression appears more invariant, suggesting that E2F activity is limited by the availability of the E2F proteins.

E2F1, a Novel Regulator of Metabolism

In the past years, several lines of evidence have shown that cell cycle regulatory proteins also can modulate metabolic processes. The transcription factor E2F1 is a central player involved in cell cycle progression, DNA-damage response, and apoptosis. Its crucial role in the control of cell fate has been extensively studied and reviewed before; however, here, we focus on the participation of E2F1 in the regulation of metabolism. We summarize recent findings about the cell cycle-independent roles of E2F1 in various tissues that contribute to global metabolic homeostasis and highlight that E2F1 activity is increased during obesity. Finally, coming back to the pivotal role of E2F1 in cancer development, we discuss how E2F1 links cell cycle progression with different metabolic adaptations required for cell growth and survival.

PARP-2 regulates cell cycle-related genes through histone deacetylation and methylation independently of poly(ADP-ribosyl)ation

Poly(ADP-ribose) polymerase-2 (PARP-2) catalyzes poly(ADP-ribosyl)ation (PARylation) and regulates numerous nuclear processes, including transcription. Depletion of PARP-2 alters the activity of transcription factors and global gene expression. However, the molecular action of how PARP-2 controls the transcription of target promoters remains unclear. Here we report that PARP-2 possesses transcriptional repression activity independently of its enzymatic activity. PARP-2 interacts and recruits histone deacetylases HDAC5 and HDAC7, and histone methyltransferase G9a to the promoters of cell cycle-related genes, generating repressive chromatin signatures. Our findings propose a novel mechanism of PARP-2 in transcriptional regulation involving specific protein-protein interactions and highlight the importance of PARP-2 in the regulation of cell cycle progression.

RAE-1 ligands for the NKG2D receptor are regulated by E2F transcription factors, which control cell cycle entry

E2F transcription factors regulate expression of RAE-1 family NKG2D ligands in cancer cells and normal proliferating cells to promote wound healing and immune recognition.

The NKG2D stimulatory receptor expressed by natural killer cells and T cell subsets recognizes cell surface ligands that are induced on transformed and infected cells and facilitate immune rejection of tumor cells. We demonstrate that expression of retinoic acid early inducible gene 1 (RAE-1) family NKG2D ligands in cancer cell lines and proliferating normal cells is coupled directly to cell cycle regulation. Raet1 genes are directly transcriptionally activated by E2F family transcription factors, which play a central role in regulating cell cycle entry. Induction of RAE-1 occurred in primary cell cultures, embryonic brain cells in vivo, and cells in healing skin wounds and, accordingly, wound healing was delayed in mice lacking NKG2D. Transcriptional activation by E2Fs is likely coordinated with posttranscriptional regulation by other stress responses. These findings suggest that cellular proliferation, as occurs in cancer cells but also other pathological conditions, is a key signal tied to immune reactions mediated by NKG2D-bearing lymphocytes.

Transcriptional repression activity of PAX3 is modulated by competition between corepressor KAP1 and heterochromatin protein 1

Pax3 is a transcription factor crucial for normal development and tumorigenesis. Pax3 has been known to cause Waardenburg syndrome and pediatric alveolar rhabdomyosarcoma, but how Pax3 regulates transcription is not clear. Here, we report that Pax3 represses transcription and selectively interacts with heterochromatin protein 1 (HP1) and KAP1. KAP1 functions as a transcriptional corepressor by recruiting HP1 to facilitate the formation of a closed chromatin through histone deacetylation and methylation. We found that KAP1 is a corepressor for Pax3 by augmenting the repressional activity of Pax3. Unexpectedly, HP1gamma diminishes the repressional activity of Pax3. On target promoters, KAP1 and HP1gamma compete for binding with Pax3 on the N-terminal paired domain, and the C-terminal domain of Pax3 governs the subcellular localization of Pax3. Taken together, our results indicate that Pax3 represses transcription through a novel mechanism involving competition between corepressor KAP1 and the heterochromatin-binding protein HP1gamma.

Differential regulation of CDP/Cux p110 by cyclin A/Cdk2 and cyclin A/Cdk1

Previous experiments with peptide fusion proteins suggested that cyclin A/Cdk1 and Cdk2 might exhibit similar yet distinct phosphorylation specificities. Using a physiological substrate, CDP/Cux, our study confirms this notion. Proteolytic processing of CDP/Cux by cathepsin L generates the CDP/Cux p110 isoform at the beginning of S phase. CDP/Cux p110 makes stable interactions with DNA during S phase but is inhibited in G2 following the phosphorylation of serine 1237 by cyclin A/Cdk1. In this study, we propose that differential phosphorylation by cyclin A/Cdk1 and cyclin A/Cdk2 enables CDP/Cux p110 to exert its function as a transcriptional regulator specifically during S phase. We found that like cyclin A/Cdk1, cyclin A/Cdk2 interacted efficiently with recombinant CDP/Cux proteins that contain the Cut homeodomain and an adjacent cyclin-binding motif (Cy). In contrast to cyclin A/Cdk1, however, cyclin A/Cdk2 did not efficiently phosphorylate CDP/Cux p110 on serine 1237 and did not inhibit its DNA binding activity in vitro. Accordingly, co-expression with cyclin A/Cdk2 in cells did not inhibit the DNA binding and transcriptional activities of CDP/Cux p110. To confirm that the sequence surrounding serine 1237 was responsible for the differential regulation by Cdk1 and Cdk2, we replaced 4 amino acids flanking the phosphorylation site to mimic a known Cdk2 phosphorylation site present in the Cdc6 protein. Both cyclin A/Cdk2 and Cdk1 efficiently phosphorylated the CDP/Cux(Cdc6) mutant and inhibited its DNA binding activity. Altogether our results help explain why the DNA binding activity of CDP/Cux p110 is maximal during S phase and decreases in G2 phase.

Lineage-specific and ubiquitous biological roles of the mammalian transcription factor LSF

Transcriptional regulation in mammalian cells is driven by a complex interplay of multiple transcription factors that respond to signals from either external or internal stimuli. A single transcription factor can control expression of distinct sets of target genes, dependent on its state of post-translational modifications, interacting partner proteins, and the chromatin environment of the cellular genome. Furthermore, many transcription factors can act as either transcriptional repressors or activators, depending on promoter and cellular contexts [Alvarez, M., Rhodes, S.J., Bidwell, J.P., 2003. Context-dependent transcription: all politics is local. Gene 313, 43-57]. Even in this light, the versatility of LSF (Late SV40 Factor) is remarkable. A hallmark of LSF is its unusual DNA binding domain, as evidenced both by lack of homology to any other established DNA-binding domains and by its DNA recognition sequence. Although a dimer in solution, LSF requires additional multimerization with itself or partner proteins in order to interact with DNA. Transcriptionally, LSF can function as an activator or a repressor. It is a direct target of an increasing number of signal transduction pathways. Biologically, LSF plays roles in cell cycle progression and cell survival, as well as in cell lineage-specific functions, shown most strikingly to date in hematopoietic lineages. This review discusses how the unique aspects of LSF DNA-binding activity may make it particularly susceptible to regulation by signal transduction pathways and may relate to its distinct biological roles. We present current progress in elucidation of both tissue-specific and more universal cellular roles of LSF. Finally, we discuss suggestive data linking LSF to signaling by the amyloid precursor protein and to Alzheimer's disease, as well as to the regulation of latency of the human immunodeficiency virus (HIV).

Transcription of dbpA, a Y box binding protein, is positively regulated by E2F1: implications in hepatocarcinogenesis

Human hepatocellular carcinoma is one of the most common cancers in the world. We previously showed that dbpA, a member of the Y box family of proteins, could accelerate the process of inflammation-induced hepatocarcinogenesis, and that dbpA is more abundantly expressed in hepatocellular carcinoma than in non-tumorous tissue. In this study, to clarify the mechanism by which expression of dbpA is enhanced in the proliferative state, we examined the transcriptional activity of the dbpA promoter region. We focused on the sequence 5'-TTTGGGGC-3' (-8 to -1 in the promoter region) resembling the E2F binding site (one base mismatch, TFSEARCH score 86.2). By overexpressing E2F1 in Huh-7 cells, transcriptional activity of dbpA was significantly increased, and this increase was abolished by mutating or deleting this sequence. Thus, expression of dbpA was positively regulated by E2F1, suggesting that one of the effects of E2F1 on cell proliferation might be mediated by dbpA at the carcinogenesis step.

A dynamic switch in Rb+/- mediated neuroendocrine tumorigenesis

Rb+/- mice develop a complex spectrum of neuroendocrine tumors on a mixed genetic (129Sv x C57BL/6) background. To understand how the 129Sv and C57BL/6 contributions affect Rb+/- tumorigenesis, we serially backcrossed Rb+/- animals to the 129Sv or C57BL/6 strain, and analysed their pathological profiles. Strikingly, the length of survival and the penetrance, severity and multiplicity of neuroendocrine tumors switch dramatically between Rb+/- animals from the two genetic backgrounds. In fact, the 129Sv background significantly enhances both the initiation and progression of tumorigenesis in the intermediate lobe of the pituitary (ILP) in Rb+/- animals. This is due to the surprising fact that ILPs from wild-type 129Sv animals are inherently abnormal, and thus greatly predisposed to neoplasia. This is likely to explain the high incidence of ILP tumors, an otherwise rare tumor type in wild-type mice, in numerous knockout studies performed on the 129Sv strain, and raises the intriguing possibility that the classic Rb+/- neuroendocrine tumors may fade away in another as of yet unidentified inbred strain. Finally, we have increased the utility of the Rb+/- tumor model, since Rb+/- animals on the C57BL/6 background develop high-penetrance tumors of the anterior lobe of the pituitary, a class of tumors estimated to occur in 20-25% of humans.

Hormone-dependent repression of the E2F-1 gene by thyroid hormone receptors

Thyroid hormone induces differentiation of many different tissues in mammals, birds, and amphibians. The different tissues all differentiate from proliferating precursor cells, and the normal cell cycle is suspended while cells undergo differentiation. We have investigated how thyroid hormone affects the expression of the E2F-1 protein, a key transcription factor that controls G1- to S-phase transition. We show that during thyroid hormone-induced differentiation of embryonic carcinoma cells and of oligodendrocyte precursor cells, the levels of E2F-1 mRNA and E2F-1 protein decrease. This is caused by the thyroid hormone receptor (TR) regulating the transcription of the E2F-1 gene. The TR binds directly to a negative thyroid hormone response element, called the Z-element, in the E2F-1 promoter. When bound, the TR activates transcription in the absence of ligand but represses transcription in the presence of ligand. In addition, liganded TR represses transcription of the S-phase-specific DNA polymerase alpha, thymidine kinase, and dihydropholate reductase genes. These results suggest that thyroid hormone-induced withdrawal from the cell cycle takes place through the repression of S-phase genes. We suggest that this is an initial and crucial step in thyroid hormone-induced differentiation of precursor cells.

Immunohistochemical expression of the transcription factor DP-1 and its heterodimeric partner E2F-1 in non-Hodgkin lymphoma

DP-1 is a G1 cell cycle-related protein that forms heterodimers with E2F, a family of transcriptional factors regulating the expression of genes important for G1 to S progression. Although the exact role of DP-1 is not well understood, it has been shown to stabilize DNA binding of E2F proteins. By immunohistochemistry, the authors examined the expression of DP-1 in lymphoid tissues, including 8 cases of reactive follicular hyperplasia and 69 cases of B-cell non-Hodgkin lymphoma. The expression of the cell cycle-related proteins E2F-1 and Ki-67 was also assessed. Scoring was based on the proportion of labeled nuclei (1-10%, 11-25%, 26-50%, and > 50%). In reactive follicular hyperplasia, staining for DP-1, E2F-1, and Ki-67 was largely confined to the germinal centers. All 25 cases of follicular lymphoma, regardless of grade, had a high proportion (> 50%) of DP-1-positive cells but a lower proportion of cells marking for E2F-1 and Ki-67 (P < 0.001). The diffuse large B-cell lymphomas (n = 24) had high DP-1 and Ki-67 scores but low E2F-1 scores (P < 0.001). Small lymphocytic (n = 10), marginal zone (n = 3), and mantle cell lymphomas (n = 5) contained relatively low proportions of cells labeled for all three markers. Precursor B-cell lymphoblastic lymphoma (n = 2) displayed high proportions of cells positive for DP-1, Ki-67, and E2F-1 (> 50% in both cases). Except in follicular center cell lesions, DP-1 expression generally correlated with that of Ki-67. However, the expression of DP-1 was discordant with that of E2F-1 in benign and malignant follicular center cells, suggesting that DP-1 may have functions other than facilitating E2F-1-dependent gene regulation and cell cycle progression in these neoplasms.

Analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced gene expression profile in vivo using pathway-specific cDNA arrays

In the current study, we used pathway-specific cDNA arrays to detect the transcriptional signature induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in vivo by studying simultaneously the expression profiles of 83 genes involved in apoptosis, cytokine production and angiogenesis. To this end, C57BL/6 mice were injected i.p. with 50 microg/kg body weight of TCDD and 1 or 3 days later, the thymus was analyzed for gene expression profiles. In the thymus, 23 out of 37 apoptotic genes screened were up-regulated by TCDD by a factor of two or more when compared to the vehicle-treated controls. In contrast, in the spleen, 20 out of 22 and in the liver, 16 out of 37 apoptotic genes were up-regulated. In the thymus, several genes encoding caspases, and members of the TNF family, including Fas ligand, were induced. Also, in the thymus, eight out of 23, and in the spleen, six out of 23 cytokine genes were up-regulated. In the liver and to a lesser extent in the thymus, certain angiogenesis genes were induced while others were repressed. When mice were injected with 0.1, 1, 10 or 50 microg/kg body weight of TCDD and the thymus was analyzed for apoptotic genes 1 day later, a dose-dependent response was not seen with most apoptotic genes. However, certain apoptotic genes were induced in the thymus even at low doses of 0.1 microg/kg body weight of TCDD. These data demonstrate that TCDD alters the expression of a large array of genes involved in apoptosis, cytokine production and angiogenesis. Thus, pathway-specific cDNA arrays may help in the identification of specific gene expression profiles induced by xenobiotics and to delineate the molecular mechanisms of toxicity.

MyoD activity upregulates E2F1 and enhances transcription from the cyclin E promoter in differentiating myoblasts lacking a functional retinoblastoma protein

We investigated the mechanism leading to cyclin E accumulation when cultured mouse myoblasts, lacking functional Rb because of sequestration or deletion, are exposed to differentiating conditions (mitogen subtraction and cell-cell contact), which activate MyoD and normally downregulate factors involved in cell division. After excluding that stabilization might account for the observed cyclin-E mRNA accumulation, we found an induction of the cyclin-E promoter that correlated with E2F activity upregulation and depended on both MyoD activation and Rb inactivation. Analyses of the E2F1-promoter activity, in normal and Rb-deficient fibroblasts converted by MyoD, identified a MyoD function stimulating E2F1 expression. The E2F1 induction was very manifest in the Rb-/- cells, but also detectable, at the early stage of differentiation, in normal cells. Its effects, although not indispensable for myogenesis, presumably contribute to raise the concentration of Rb-E2F1 transcription-repressing complexes, since MyoD strongly induces also Rb in differentiating myocytes. The activity of an E2F1 promoter lacking the E2F sites indicated that E2F1 itself underwent self-repression by such mechanism at late stages of differentiation. In the absence of Rb, however, the induced E2F1 is left with only its activating role, reversing the normal effect of this MyoD function.

The influence of INK4 proteins on growth and self-renewal kinetics of hematopoietic progenitor cells

This study investigated the influence of expression of proteins of the INK4 family, particularly p16, on the growth and self-renewal kinetics of hematopoietic cells. First, retrovirus-mediated gene transfer (RMGT) was used to restore p16(INK4a) expression in the p16(INK4a)-deficient lymphoid and myeloid cell lines BV173 and K562, and it was confirmed that this inhibited their growth. Second, to sequester p16(INK4a) and related INK4 proteins, cyclin-dependent kinase 4 (CDK4) was retrovirally transduced into normal human CD34(+) bone marrow cells and then cultured in myeloid colony-forming cell (CFC) assays. The growth of CDK4-transduced colonies was more rapid; the cell-doubling time was reduced; and, upon replating, the colonies produced greater yields of secondary colonies than mock-untransduced controls. Third, colony formation was compared by marrow cells from p16(INK4a-/-) mice and wild-type mice. The results from p16(INK4a-/-) marrow were similar to those from CDK4-transduced human CFCs, in terms of growth rate and replating ability, and were partially reversed by RMGT of p16(INK4a). Lines of immature granulocytic cells were raised from 15 individual colonies grown from the marrow of p16(INK4a-/-) mice. These had a high colony-forming ability (15%) and replating efficiency (96.7%). The p16(INK4a-/-) cell lines readily became growth factor-independent upon cytokine deprivation. Taken together, these results demonstrate that loss of INK4 proteins, in particular p16(INK4a), increases the growth rate of myeloid colonies in vitro and, more importantly, confers an increased ability for clonal expansion on hematopoietic progenitor cells.

Cell cycle basis for the onset and progression of c-Myc-induced, TGFalpha-enhanced mouse mammary gland carcinogenesis

Using single and double transgenic mouse models, we investigated how c-Myc modulates the mammary epithelial cell cycle to induce cancer and how TGFalpha enhanced the process. In c-myc transgenic mice, c-myc expression was high in the hyperplastic mammary epithelium and in the majority of tumor areas. However, the tumors displayed focal areas of low expression of c-myc but high rates of proliferation. In contrast to E2F1 and cyclin A2, which were induced and co-localized with c-myc expression, induction of cyclins D1 and E occurred only in these tumor foci. Overexpression of cyclin D1 also occurred in the hyperplastic epithelium of tgfalpha-single and tgfalpha/c-myc-double transgenic mice. In tgfalpha/c-myc tumors, cells positive for cyclins D1 and E were randomly spread, without showing a reciprocal relationship to c-myc expression. In contrast to c-myc tumors, most tgfalpha/c-myc tumors showed undetectable levels of retinoblastoma protein (pRB), and the loss of pRB occurred in some cases at the mRNA level. These results suggest that E2F1 and cyclin A2 may be induced by c-Myc to mediate the onset of mammary cancer, whereas overexpression of cyclins D1 and E may occur later to facilitate tumor progression. TGFalpha may play its synergistic role, at least in part, by inducing cyclin D1 and facilitating the loss of pRB.

Stable expression of a dominant negative mutant of CCAAT binding factor/NF-Y in mouse fibroblast cells resulting in retardation of cell growth and inhibition of transcription of various cellular genes

The heterotrimeric CCAAT-binding factor CBF specifically interacts with the CCAAT motif present in the proximal promoters of numerous mammalian genes. To understand the in vivo function of CBF, a dominant negative mutant of CBF-B subunit that inhibits DNA binding of wild type CBF was stably expressed in mouse fibroblast cells under control of tetracycline-responsive promoter. Expression of the mutant CBF-B but not the wild-type CBF-B resulted in retardation of fibroblast cell growth. The analysis of cell growth using bromodeoxyuridine labeling showed that expression of the mutant CBF-B decreased the number of cells entering into S phase, and also delayed induction of S phase in the quiescent cells after serum stimulation, thus indicating that the inhibition of CBF binding prolonged the progression of S phase in fibroblasts. These results provide direct evidence for the first time that CBF is an important regulator of fibroblast growth. The inhibition of CBF binding reduced expression of various cellular genes including the alpha2(1) collagen, E2F1, and topoisomerase IIalpha genes which promoters contain the CBF-binding site. This result implied that expression of many other genes which promoters contain CBF-binding site was also decreased by the inhibition of CBF binding, and that the decreased expression of multiple cellular genes possibly caused the retardation of fibroblast cell growth.

Activation of the murine dihydrofolate reductase promoter by E2F1. A requirement for CBP recruitment

The E2F family of heterodimeric transcription factors plays an important role in the regulation of gene expression at the G1/S phase transition of the mammalian cell cycle. Previously, we have demonstrated that cell cycle regulation of murine dihydrofolate reductase (dhfr) expression requires E2F-mediated activation of the dhfr promoter in S phase. To investigate the mechanism by which E2F activates an authentic E2F-regulated promoter, we precisely replaced the E2F binding site in the dhfr promoter with a Gal4 binding site. Using Gal4-E2F1 derivatives, we found that E2F1 amino acids 409-437 contain a potent core transactivation domain. Functional analysis of the E2F1 core domain demonstrated that replacement of phenylalanine residues 413, 425, and 429 with alanine reduces both transcriptional activation of the dhfr promoter and protein-protein interactions with CBP, transcription factor (TF) IIH, and TATA-binding protein (TBP). However, additional amino acid substitutions for phenylalanine 429 demonstrated a strong correlation between activation of the dhfr promoter and binding of CBP, but not TFIIH or TBP. Finally, transactivator bypass experiments indicated that direct recruitment of CBP is sufficient for activation of the dhfr promoter. Therefore, we suggest that recruitment of CBP is one mechanism by which E2F activates the dhfr promoter.

Characterization of the 3' untranslated region of mouse E2F1 mRNA

The E2F family of transcription factors has been implicated in the regulation of the G1 to S phase transition of the mammalian cell cycle. We have focused on characterizing the cell cycle stage-specific expression of one family member, E2F1. Previous studies indicated that there are two mouse E2F1 (mE2F1) mRNA species whose abundance peaks in early S phase. However, it was unknown as to what constituted the structural difference between the two mE2F1 mRNAs and whether or not they encoded identical proteins. We have now cloned sequences corresponding to the 3' untranslated region (3'-UTR) of the mE2F1 gene. Northern blot analyses using different probes demonstrated that the two E2F1 mRNAs were distinguished by differences in the length of their 3' UTRs. We found that the longer (2.7-kb) mE2F1 mRNA contained two consensus RNA instability elements that the shorter (2.2-kb) mE2F1 mRNA lacked. However, a comparison of the stability of the 2.7-kb and the 2.2-kb mE2F1 mRNAs suggests that both mE2F1 mRNAs are fairly stable, having a half-life of 6-9h in both asynchronously growing cells and in the S phase of synchronized cells. Thus, we have determined that both mE2F1 mRNAs contain the identical coding region of the E2F1 protein and that enforced expression of mE2F1 mRNA should not be hampered by problems with RNA stability.

Antisense nucleic acids targeted to the thymidylate synthase (TS) mRNA translation start site stimulate TS gene transcription

Thymidylate synthase (TS) is a key enzyme in the synthesis of DNA and a target for cancer chemotherapeutic agents. Antisense TS nucleic acids may be useful in enhancing anticancer drug effectiveness. MCF-7 and HeLa cells were transfected with vectors expressing antisense TS RNA or with antisense oligodeoxynucleotides (AS-ODNs) to different TS mRNA regions. Antisense RNAs were targeted to 30 bases of the TS mRNA including part of the stem loop at the translation start site and to 30 bases spanning the exon1/exon2 boundary. AS-ODNs were targeted to the translation start site and the translation stop site. Antisense nucleic acids complementary to the translation start site (and not the exon1/exon2 boundary or translation stop site) significantly enhanced constitutive TS gene transcription. Therefore, TS mRNA sequences appear to be involved in a novel pathway controlling TS gene transcription. Induced transcription could hinder antisense-based attempts to inhibit TS and must be considered when designing such strategies.

Modulation of E2F activity via signaling through surface IgM and CD40 receptors in WEHI-231 B lymphoma cells

Stimulation of the phenotypically immature B cell lymphoma WEHI-231 with anti-IgM induces G1 arrest followed by apoptotic cell death, which can be reversed by stimulation via the CD40 receptor. Here, we show that cells expressing bcl-xL (WEHI-bcl-xL) arrest at G0/G1 following culture with anti-IgM but do not undergo apoptosis. These arrested cells can be induced to reenter the cell cycle by ligation of CD40. We have therefore used these cells as a model to study the regulation of the transcription factor E2F, which is critically involved in transit through the cell cycle. We found that anti-IgM treatment induces the appearance of an inhibitory DNA binding complex containing the pRB-related pocket protein p130 together with E2F and a concomitant decrease in "free" E2F, consisting of E2F1 and its partner DP1; these effects were reversed following stimulation via CD40. These changes in free E2F levels were regulated by changes in E2F1 gene transcription, which is at least partly a result of control of E2F1 promoter activity through its E2F binding sites. Transient transfection experiments showed that either E2F1 or the viral oncoprotein E1A, which sequesters pocket proteins, including p130, overcame anti-IgM-induced cell cycle arrest in WEHI-bcl-xL. Taken together, these results indicate that in WEHI-231 sIgM ligation induces the accumulation of hypophosphorylated p130 with consequent inhibition of E2F1 gene transcription and cell cycle arrest. Conversely, ligation of CD40 causes hyperphosphorylation of p130, thereby releasing the repression of E2F1 and other E2F-regulated genes, enabling the cells to reenter the cycle. These results, therefore, provide novel insights into the mechanisms whereby antigen receptors on immature B cells deliver inhibitory signals (leading to negative selection of self-reactive B cells) and how these signals can be modulated by positive signals generated via CD40.

Necdin, a postmitotic neuron-specific growth suppressor, interacts with viral transforming proteins and cellular transcription factor E2F1

Necdin is a nuclear protein expressed in virtually all postmitotic neurons, and ectopic expression of this protein strongly suppresses the proliferation of NIH3T3 cells. Simian virus 40 large T antigen targets both p53 and the retinoblastoma protein (Rb) for cellular transformation. By analogy with the interactions of the large T antigen with these nuclear growth suppressors, we examined the ability of necdin to bind to the large T antigen. Necdin was co-immunoprecipitated with the large T antigen from the nuclear extract of necdin cDNA-transfected COS-1 cells. Yeast two-hybrid and in vitro binding analyses revealed that necdin bound to an amino-terminal region of the large T antigen, which encompasses the Rb-binding domain. Moreover, necdin bound to adenovirus E1A, another viral oncoprotein that forms a specific complex with Rb. We then examined the ability of necdin to bind to the transcription factor E2F1, a cellular Rb-binding factor involved in cell-cycle progression. Intriguingly, necdin, like Rb, bound to a carboxyl-terminal domain of E2F1, and repressed E2F-dependent transactivation in vivo. In addition, necdin suppressed the colony formation of Rb-deficient SAOS-2 osteosarcoma cells. These results suggest that necdin is a postmitotic neuron-specific growth suppressor that is functionally similar to Rb.

Carbon monoxide controls the proliferation of hypoxic vascular smooth muscle cells

Excess vascular smooth muscle cell (VSMC) proliferation and contractility are key events in the pathophysiology of vascular disorders induced by hypoxia. We have recently reported that carbon monoxide (CO), produced by VSMC under conditions of hypoxia, can be a modulator of cGMP levels in both endothelial and smooth muscle cells. In this respect, some of the physiologic effects of CO in the vasculature parallel those of nitric oxide (NO), a well characterized regulator of vascular tone. We report here that under hypoxia, VSMC-derived CO is an important regulator of VSMC proliferation. Inhibiting CO formation or scavenging CO with hemoglobin increased VSMC proliferation in response to serum or to mitogens such as endothelin, whereas increasing CO production or exposing cells to exogenous CO lead to a markedly attenuated growth response. The effects of CO on VSMC proliferation correlated with changes in E2F-1 expression, the prototype member of a family of transcription factors that participate in the control of cell cycle progression. CO significantly suppressed E2F-1 expression, whereas, removal of CO from the cultures with hemoglobin lead to increased E2F-1 gene transcription, mRNA, and protein production as well as mRNA levels of c-myc, a target gene of E2F-1. Moreover, the actions of CO were mediated by the second messenger molecule, cGMP. Limiting VSMC growth by increasing the release of CO may represent a key event in the body's compensatory responses to hypoxia.

Cell-growth regulation of the hamster dihydrofolate reductase gene promoter by transcription factor Sp1

The dihydrofolate reductase (DHFR) gene (dhfr) promoter contains cis-acting elements for the transcription factors Sp1 and E2F. Given the ability of Sp1 to activate the dhfr promoter, we have evaluated the contribution of Sp1 to the cell-growth regulation of the dhfr gene. Using gel-mobility assays performed with DNA probes from the minimal promoter of the hamster dhfr gene and nuclear extracts from cultured hamster cells (CHO K1) we show that the binding of Sp1 to the dhfr promoter is cell-growth-phase regulated. Accordingly, dhfr transcription and mRNA levels in K1 cells increase upon serum stimulation. Cytological detection of Sp1 by immunofluorescence reveals a decrease of this protein in the process leading to the G0 state, and an increase upon serum stimulation of quiescent cells. These results were confirmed by western blot analysis. It is concluded that Sp1 progressively binds to the hamster dhfr promoter after stimulation of cell proliferation, which can account for the transcriptional regulation of the dhfr gene during the cell cycle. The role of Sp1 in the specific control of dhfr during the cell cycle was confirmed in vivo using cell lines derived from dhfr-negative cells transfected with dhfr plasmids carrying either the wild-type or mutated Sp1-binding or E2F-binding sites in the dhfr minimal promoter.

Expression patterns of the E2F family of transcription factors during mouse nervous system development

The E2F family of transcription factors consists of two subgroups termed E2F and DP. E2F is required for cell proliferation, and is necessary for fruit fly development. E2F activity is a target for regulation by the retinoblastoma gene family, which includes pRB, p107 and p130. Mutant RB-/-, RB-/-:p107-/- and p107-/-:p130-/- mice develop abnormally, probably as a result of dysregulation in the activity of E2F, indicating the importance of E2F in mammalian development. To investigate the role of E2F in murine development, we have examined the patterns of expression of E2F-1 through E2F-5, and DP-1 in the developing nervous system by in situ hybridization. E2F-1, E2F-2 and E2F-5 are first detected in the 9.5 days post-coitus (dpc) forebrain. Expression of these E2F forms extends caudally thereafter and includes the developing brain and the upper half of the 10.5 dpc spinal cord. By 11.5 dpc, these E2F factors are expressed throughout the central nervous system. In 12.5 dpc embryos, E2F-1, E2F-2 and E2F-5 are highly expressed in proliferating, undifferentiated neuronal precursors. As neurons differentiate and migrate to the outer marginal zones in the nervous system, expression of these E2F members is extinguished. In the developing retina, another neuronal tissue, E2F-1 expression is also confined to the proliferating, undifferentiated retinoblastic layer. In contrast, E2F-3 expression is up-regulated as retinoblasts differentiate into the ganglion cell layer. In non-neuronal tissues, high E2F-4 transcript levels are present in regions corresponding to proliferative chondrocytes, whereas E2F-2 and E2F-4 transcripts are very abundant in the thymic cortex, which contains immature thymocytes. We conclude that individual E2F forms are differentially regulated during the development of distinct tissues, and especially during neuronal development.

Cellular targets for activation by c-Myc include the DNA metabolism enzyme thymidine kinase

Although a remarkable number of genes has been identified that are either activated or repressed via c-Myc, only few of them obviously contribute to Myc's biological effect--the induction of proliferation. We found that in logarithmically growing cells overexpression of Myc specifically induces thymidine kinase (TK) mRNA expression and enzyme activity, whereas loss of one allele of Myc causes downregulation of this enzyme. We show that activation of Myc triggers high levels of this normally strictly S-phase-regulated DNA metabolism enzyme in serum arrested G0 cells and causes high and constant levels of TK expression throughout the entire ongoing cell cycle. Induction of TK by Myc requires an intact transcriptional activation domain. Myc-induced deregulation of this enzyme is paralleled by alterations of protein binding at the E2F-site of the TK promoter. We further show that cell growth arrest by the cyclin-dependent kinase inhibitor p16 is abrogated by overexpression of Myc and that co-overexpression of p16 cannot inhibit the Myc-induced up-regulation of TK expression. Our data demonstrate TK to be a cellular target of Myc independently of the status of cell proliferation and provide evidence that the transcription factor E2F might be involved in this process.

Use of normal and transgenic mice to examine the relationship between terminal differentiation of intestinal epithelial cells and accumulation of their cell cycle regulators

A spatially well organized continuum of proliferation, differentiation, and death is displayed along crypt-villus units in the adult mouse small intestine. This continuum provides an opportunity to examine in vivo the mechanisms by which proliferative status changes as a function of cellular differentiation. Immunohistochemical studies of normal FVB/N mice revealed that as epithelial cells complete their terminal differentiation during a 48-72-h migration up villi, there is a marked and rapid fall in the levels of two important regulators of the G1/S transition, cyclin D1 and cyclin-dependent kinase (cdk) 2. However, cellular levels of their partners, cdk4 and cyclin E, remain unchanged as does the level of pRB. Adult FVB/N transgenic mice were studied that contained an intestinal fatty acid binding protein gene promoter (Fabpi) linked to wild type Simian virus 40 large T antigen (SV40 TAgWt) or a mutant TAg with Lys for Glu substitutions at residues 107 and 108 (SV40 TAgK107/8) that fails to bind pRB and related pocket proteins. Both transgenes are expressed only in villus enterocytes. SV40 TAgWt causes these terminally differentiated cells to re-enter the cycle. Re-entry is accompanied by a reduction in un/hypophosphorylated pRB, an induction of cyclin D1 and cdk2, but no change in cdk4, cyclin E, or E2F-1. In contrast, SV40 TAgK107/8 fails to induce re-entry and does not produce changes in un/hypophosphorylated pRB, cyclin D1, or cdk2 accumulation. These results suggest that un/hypophosphorylated pRB is an important mediator of the cell cycle arrest that normally occurs as enterocytes exit the crypt and complete their differentiation. Fabpi-directed expression of E2F-1 does not cause villus enterocytes to return to the cell cycle, alter their suppression of cyclin D1 or cdk2, or affect their state of differentiation, emphasizing the insensitivity of these cells to the effects of E2F-1. Analyses of p53(-/-) and p53(+/+) mice containing Fabpi-SV40 TAgWt and Fabpi-SV40 TAgK107/8 established that the proliferation induced by SV40 TAgWt does not require p53 and is associated with increased (p53-independent) apoptosis. The presence of cyclin E and cdk4 in differentiating villus enterocytes emphasizes that these cells retain part of their proliferative heritage expressed 24-72 h earlier in the crypt. The data suggest that down-regulation of cdk2 and/or cyclin D1 expression may be important for control of proliferative status and/or execution of terminal differentiation.

Nuclear localization of DP and E2F transcription factors by heterodimeric partners and retinoblastoma protein family members

E2F is a family of transcription factors implicated in the regulation of genes required for progression through G1 and entry into the S phase. The transcriptionally active forms of E2F are heterodimers composed of one polypeptide encoded by the E2F gene family and one polypeptide encoded by the DP gene family. The transcriptional activity of E2F/DP heterodimers is influenced by association with the members of the retinoblastoma tumor suppressor protein family (pRb, p107, and p130). Here the intracellular distribution of E2F and DP proteins was investigated in transiently transfected Chinese hamster and human cells. In transfected cells, DP-1 did not accumulate in the nucleus unless it was coexpressed with the heterodimeric partners E2F-1, E2F-2, or E2F-3. Domain mapping experiments showed that regions of E2F-1 and DP-1 that are required for stable association of the two proteins were also required for nuclear localization of DP-1. Unlike E2F-1, -2, and -3, E2F-4 did not accumulate in the nucleus unless it was coexpressed with DP-2, p107 and p130, but not pRb, stimulated nuclear localization of E2F-4, either alone or in combination with DP-2. These results indicate that DP proteins preferentially associate with specific E2F partners, and suggest that the ability of specific E2F/DP heterodimers to localize in the nucleus contributes to the regulation of E2F activity.

E2F-1 functions in mice to promote apoptosis and suppress proliferation

Members of the E2F transcription factor family (E2F-1-E2F-5) are believed to be critical positive regulators of cell cycle progression in eukaryotes although the in vivo functions of the individual E2Fs have not been elucidated. Mice were generated that lack E2F-1 and, surprisingly, these mice develop and reproduce normally. However, E2F-1-/- mice exhibit a defect in T lymphocyte development leading to an excess of mature T cells due to a maturation stage-specific defect in thymocyte apoptosis. As E2F-1-/- mice age they exhibit a second phenotype marked by aberrant cell proliferation. These findings suggest that while certain members of the E2F family may positively regulate cell cycle progression, E2F-1 functions to regulate apoptosis and to suppress cell proliferation.

Tumor induction and tissue atrophy in mice lacking E2F-1

The retinoblastoma tumor suppressor protein (pRB) is a transcriptional repressor that regulates gene expression by physically associating with transcription factors such as E2F family members. Although pRB and its upstream regulators are commonly mutated in human cancer, the physiological role of the pRB-E2F pathway is unknown. To address the function of E2F-1 and pRB/E2F-1 complexes in vivo, we have produced mice homozygous for a nonfunctional E2F-1 allele. Mice lacking E2F-1 are viable and fertile, yet experience testicular atrophy and exocrine gland dysplasia. Surprisingly, mice lacking E2F-1 develop a broad and unusual spectrum of tumors. Although overexpression of E2F-1 in tissue culture cells can stimulate cell proliferation and be oncogenic, loss of E2F-1 in mice results in tumorigenesis, demonstrating that E2F-1 also functions as a tumor suppressor.

Molecular cloning, chromosomal mapping, and expression of the mouse p107 gene

Progression through the G1 phase of the cell cycle is regulated, in part, by the pRB-family proteins, pRB and p107. The basis for this regulation is due to a network of interactions between the pRB-family proteins, pRB, p107, and p130; the E2F-family of transcription factors; and cyclins D, E, and A. One of the pRB-family proteins, p107, has also been found to bind to the transactivation domain of the c-Myc proto-oncogene. This region in c-Myc is frequently mutated in tumors such as Burkitt's lymphoma, HIV-associated lymphoma, and multiple myeloma. The binding of p107 and regulation of c-Myc may conceivably be disrupted not only by mutations in c-Myc, but possibly by mutations in p107. In order to determine if mutations in p107 are indeed present in mouse B-cell tumors which exhibit a lower frequency of c-Myc mutation, we have cloned the mouse p107 cDNA and compared this sequence with its human counterpart. We find that the extreme N-terminal and C-terminal regions are the most conserved between human and mouse p107 sequences. Chromosomal positioning of the locus for p107 (designated Rbl1) as well as E2f1 to the distal end of mouse Chromosome (Chr) 2 also suggests a close but unlinked genetic relationship between these cell cycle regulatory transcription factors.

Transcriptional regulation of the dihydrofolate reductase gene

As cells approach S phase, many changes occur to create an environment conducive for DNA synthesis and commitment to cell division. The transcription rate of many genes encoding enzymes involved in DNA synthesis, including the dihydrofolate reductase (dhfr) gene, increases at the G1/S boundary of the cell cycle. Although a number of transcription factors interact to finely tune the levels of dhfr RNA produced, two families of transcription factors, Sp1 and E2F, play central roles in modulating dhfr levels. A region containing several Sp1-binding sites is required for both regulated and basal transcription levels. In contrast, the E2F-binding sites near the transcription start site are required only for regulated transcription. A model is presented for the regulation of the dhfr gene which may also pertain to other cell cycle-associated genes.

Loss of the p16/MTS1 tumor suppressor gene causes E2F-mediated deregulation of essential enzymes of the DNA precursor metabolism

Homozygous deletions of the tumor suppressor gene p16/MTS1 were reported in a wide variety of tumors and tumor cell lines. Its product inhibits the phosphorylation of the retinoblastoma protein (pRb) by CDK4 and CDK6. Because phosphorylation of pRb is a major regulatory event in the activation of the transcription factor E2F, a role for p16 in the regulation of E2F-dependent transcription was presumed. We investigated the effect of the loss of p16 on E2F-mediated transcription in a tumor progression model consisting of three cell lines originating from a common precursor cell--one p16-positive cell line established from the primary biopsy and two lines derived from more advanced stages of the tumor representing the same cell clone after loss of p16. We observed up- and deregulation of E2F-dependent transcription during the cell cycle of the p16-negative cell clones, which returned to normal after transient expression of p16. This p16-dependent regulation affects a set of enzymes necessary for the activation of all four DNA precursors; it is paralleled by the interconversion of transcriptionally active free E2F and transcriptionally inactive higher molecular complexes of E2F and is dependent on the existence of endogenous pRb. Furthermore, we show that p16-negative cell clones exhibit a growth advantage compared to their p16-positive counterparts. One might speculate that one feature of tumor progression could be deregulation of E2F-dependent transcription caused by loss of p16.

The cellular effects of E2F overexpression

The product of the retinoblastoma tumor-suppressor gene (RB) is a ubiquitously expressed, 105-kDa nuclear phosphoprotein (pRB). The pRB protein negatively regulates the cellular G1/S phase transition, and it is at this point in the cell cycle that it is thought to play its role as a tumor suppressor. The growth-inhibitory effects of pRB are exerted, at least in part, through the E2F family of transcription factors. This chapter reviews the insights into the mechanism of action of the E2F family members that have been obtained through overexpression studies. Studies in RB-/- SAOS-2 cells have provided evidence in support of the hypothesis that the E2F family members are negatively regulated by pRB and the related protein p130. In particular, the results obtained are consistent with the earlier biochemical data which suggested that E2F1 is regulated primarily by pRB, and E2F4 by p130. Results relating to p107 are also discussed. Consistent with the proposed role of pRB and E2F1 as coregulators of entry into S phase, experiments have demonstrated that overexpression of E2F1 is sufficient to override the cell cycle arrests caused by serum deprivation of fibroblasts or transforming growth factor-beta (TGF-beta) treatment of mink lung epithelial cells. However, at least in the case of the serum deprivation induced arrest, the ultimate result of E2F1 overexpression is death by p53-dependent apoptosis. In light of this and other data, a model is discussed as to how functional inactivation of pRB and p53 might cooperate to promote tumorigenesis. A number of studies have demonstrated the oncogenic potential of E2F family members, at least under certain conditions. This is, again, in keeping with the notion that these proteins play a critical role in controlling proliferation.

Constitutive expression of the E2F1 transcription factor in fibroblasts alters G0 and S phase transit following serum stimulation

The E2F1 transcription factor was constitutively expressed in NIH3T3 fibroblasts to determine its effect on the cell cycle. These E2F1 cell lines were not tightly synchronized in G0 phase of the cell cycle following serum starvation, as are normal fibroblasts. Instead, the cells are spread throughout G0 and G1 phase with a portion of the population initiating DNA synthesis. Upon serum stimulation, the remaining cells in G0/G1 begin to enter S phase immediately but with a reduced rate. Constitutive expression of E2F1 appears to primarily affect the G0 phase, since transit of proliferating E2F1 cell lines through G1 phase is the same as control cells. Consistent with a shortened G0 phase, the E2F1 cell lines have a significantly reduced cellular volume. Additionally, the first S phase after serum stimulation, but not subsequent S phases, is nearly doubled in the E2F1 cell lines compared with control cells. Cell lines expressing a deletion mutant of E2F1 (termed E2F1d87), known to significantly affect cell shape, have cell cycle and volume characteristics similar to the E2F1 expressing cells. However, all S phase durations are considerably lengthened and the cells demonstrate delayed growth after plating.

Introduction to the E2F family: protein structure and gene regulation

E2F is a heterodimer composed of two partners, such as E2F1 and DP1. Although E2F1 can bind DNA as a homodimer and increase promoter activity, optimal DNA-binding and transcriptional activity occurs in the heterodimeric form. A model (Fig. 3) for the involvement of E2F activity in cell growth control that incorporates viral oncoproteins, positive regulators of cell growth (cyclins) and negative regulators of cell growth (tumor suppressor proteins) can now be advanced. Each aspect of this model is addressed in subsequent chapters of this book. It is likely that binding of growth-suppressing proteins, such as Rb, can inhibit the transactivation potential of E2F1, either by blocking the interaction of E2F1 with a separate component of the transcription complex or by bringing a repressor domain to the transcription complex (Flemington et al. 1993; Helin et al. 1993; Weintraub et al. 1992; Zamanian and La Thangue 1993; Zhu et al. 1993). Phosphorylation or sequestration of Rb by viral oncoproteins can free E2F. The influence of viral oncoproteins on E2F activity and the regulation of the different E2F complexes is the focus of the contributions by Cobrinik and by Cress and Nevens. The interaction of the free E2F induces a bend in the DNA that may also play a role in transactivation, perhaps by bringing proteins (such as an Sp1 or CCAAT family member) separated by distance on the promoter DNA into contact (Huber et al. 1994). Because E2F target genes encode proteins critical for cell growth, deregulation of E2F activity can have severe consequences, such as apoptosis or uncontrolled proliferation. The effect of altered expression of E2F activity on the cell cycle and on tumorigenicity is the focus of the contribution by Adams and Kaelin. Finally, a comparison of E2F to the genetically well-characterized factors that regulate G1/S phase transcription in yeast is the subject of the chapter by Breeden. This volume concludes with Farnham's summary of the rapid gains in knowledge concerning the E2F gene family that have been made in the past several years and provides a series of questions and lines of investigation that will be the focus of future studies.

Thirteen genes (Cebpb, E2f1, Tcf4, Cyp24, Pck1, Acra4, Edn3, Kcnb1, Mc3r, Ntsr, Cd40, Plcg1 and Rcad) that probably lie in the distal imprinting region of mouse chromosome 2 are not monoallelically expressed

Seven imprinted genes are currently known in the mouse but none have been identified yet in the distal imprinting region of mouse Chromosome (Chr) 2, a region which shows striking linkage conservation with human chromosome 20q13. Both maternal duplication/paternal deficiency and its reciprocal for distal Chr 2 lead to mice with abnormal body shapes and behavioural abnormalities. We have tested a number of candidate genes, that are either likely or known to lie within the distal imprinting region, for monoallelic expression. These included 3 genes (Cebpb, E2f1 and Tcf4) that express transcription factors, 2 genes (Cyp24 and Pck1) that are involved in growth, 5 genes (Acra4, Edn3, Kcnb1, Mc3r and Ntsr) where a defect could lead to neurological and probably behavioural problems, and 3 genes (Cd40, Plcg1 and Rcad) that are less obvious candidates but sequence information was available for designing primers to test their expression. On/off expression of each gene was tested by reverse transcription-polymerase chain reaction (RT-PCR) analysis of RNA extracted from tissues of mice with maternal duplication/paternal deficiency and its reciprocal for the distal region of Chr 2. None of the 13 genes is monoallelically expressed in the appropriate tissues before and shortly after birth which suggests that these genes are not imprinted later in development. This study has narrowed down the search for imprinted genes, and valuable information on which genes have been tested for on/off expression is provided. Since there is considerable evidence of conservation of imprinting between mouse and human, we would predict that the 13 genes are not imprinted in human. Five of the genes: E2f1, Tcf4, Kcnb1, Cd40 and Rcad, have not yet been mapped in human. However, because of the striking linkage conservation observed between mouse Chr 2 and human chromosome 20, we would expect these genes to map on human chromosome 20q13.

DP and E2F proteins: coordinating transcription with cell cycle progression

Transcriptional control during the G1/S transition is important in regulating cell cycle progression. The transcription factor DRTF1/E2F is believed to play a crucial role in this process by integrating the activity of the machinery that drives the cell cycle with the transcription apparatus. Being the point of convergence for growth-promoting and growth-inhibitory signals, it is a pivotal cellular target for molecules which subvert normal cell cycle control, such as oncoviral proteins. Recent studies have indicated that members of two distinct families of proteins, DP and E2F, interact combinatorially as DP/E2F heterodimers in DRTF1/E2F. The activities of both DP and E2F proteins are under cell cycle control, being influenced by the level of phosphorylation imparted through the cell cycle regulated activity of cyclin-dependent kinases. Both DP and E2F proteins are endowed with proto-oncogenic activity and, conversely, have been implicated in regulating apoptosis. Current evidence suggests therefore that the activity of DRTF1/E2F is instrumental in regulating progression through the cell cycle.