Transcriptional repression of the E2F-1 gene by interferon-alpha is mediated through induction of E2F-4/pRB and E2F-4/p130 complexes

Increased expression of HPV-E7 oncoprotein correlates with a reduced level of pRb proteins via high viral load in cervical cancer

Human Papillomavirus (HPV) is the most common cause of sexually transmitted diseases and causes a wide range of pathologies including cervical carcinoma. Integration of the HR-HPV DNA into the host genome plays a crucial role in cervical carcinoma. An alteration of the pRb pathways by the E7 proteins is one of the mechanisms that's account for the transforming capacity of high-risk papillomavirus. For the proper understanding of the underline mechanism of the progression of the disease, the present study investigate the correlation of concentration of host pRb protein, viral E7 oncoprotein and viral load in early and advanced stages of cervical carcinoma. It was found that the viral load in early stages (stage I and II) was less (log10 transformed mean value 2.6 and 3.0) compared to advanced stages (stage III and IV) (Log10 transformed value 5.0 and 5.8) having high expression of HPV E7 onco-protein and reduced level of pRb protein, signifying the role of viral load and expression level of E7 oncoprotein in the progression of cervical cancer.

E2F1 transcription factor and its impact on growth factor and cytokine signaling

E2F1 is a transcription factor involved in cell cycle regulation and apoptosis. The transactivation capacity of E2F1 is regulated by pRb. In its hypophosphorylated form, pRb binds and inactivates DNA binding and transactivating functions of E2F1. The growth factor stimulation of cells leads to activation of CDKs (cyclin dependent kinases), which in turn phosphorylate Rb and hyperphosphorylated Rb is released from E2F1 or E2F1/DP complex, and free E2F1 can induce transcription of several genes involved in cell cycle entry, induction or inhibition of apoptosis. Thus, growth factors and cytokines generally utilize E2F1 to direct cells to either fate. Furthermore, E2F1 regulates expressions of various cytokines and growth factor receptors, establishing positive or negative feedback mechanisms. This review focuses on the relationship between E2F1 transcription factor and cytokines (IL-1, IL-2, IL-3, IL-6, TGF-beta, G-CSF, LIF), growth factors (EGF, KGF, VEGF, IGF, FGF, PDGF, HGF, NGF), and interferons (IFN-α, IFN-β and IFN-γ).

IFN-α inhibits telomerase in human CD8⁺ T cells by both hTERT downregulation and induction of p38 MAPK signaling

The cytokine IFN-α is secreted during viral infections and has been shown to inhibit telomerase activity and accelerate T cell differentiation in vivo. However, the mechanism for this inhibition is not clear. In this study, we show that IFN-α inhibits both the transcription and translation of human telomerase reverse transcriptase (hTERT), the catalytic component of telomerase, in activated CD8(+) T cells. This was associated with increased activity of the repressor of hTERT transcription E2 transcription factor and decreased activation of NF-κB that promotes hTERT transcription. However IFN-α did not affect the translocation of hTERT from the cytoplasm to the nucleus. IFN-α also inhibits AKT kinase activation but increases p38 MAPK activity, and both of these events have been shown previously to inhibit telomerase activity. Addition of BIRB796, an inhibitor of p38 activity, to IFN-α-treated cells reversed, in part, the inhibition of telomerase by this cytokine. Therefore, IFN-α can inhibit the enzyme telomerase in CD8(+) T cells by transcriptional and posttranslational mechanisms. Furthermore, the addition of IFN-α to CD8(+)CD27(+)CD28(+) T cells accelerates the loss of both these costimulatory molecules. This suggests that persistent viral infections may contribute to the accumulation of highly differentiated/senescent CD8(+)CD27(-)CD28(-) T cells during aging by promoting IFN-α secretion during repeated episodes of viral reactivation.

Type I interferons induce apoptosis by balancing cFLIP and caspase-8 independent of death ligands

Interferons induce a pleiotropy of responses through binding the same cell surface receptor. Here we investigated the molecular mechanism driving interferon-induced apoptosis. Using a nonbiased small interfering RNA (siRNA) screen, we show that silencing genes whose products are directly engaged in the initiation of interferon signaling completely abrogate the interferon antiproliferative response. Apoptosis-related genes such as the caspase-8, cFLIP, and DR5 genes specifically interfere with interferon-induced apoptosis, which we found to be independent of the activity of death ligands. The one gene for which silencing resulted in the strongest proapoptotic effect upon interferon signaling is the cFLIP gene, where silencing shortened the time of initiation of apoptosis from days to hours and increased dramatically the population of apoptotic cells. Thus, cFLIP serves as a regulator for interferon-induced apoptosis. A shift over time in the balance between cFLIP and caspase-8 results in downstream caspase activation and apoptosis. While gamma interferon (IFN-γ) also causes caspase-8 upregulation, we suggest that it follows a different path to apoptosis.

E2F4 plays a key role in Burkitt lymphoma tumorigenesis

Sporadic Burkitt lymphoma (sBL) is a rapidly growing B-cell non-Hodgkin's lymphoma whose treatment requires highly aggressive therapies that often result severely toxic. Identification of proteins whose expression or function is deregulated in sBL and play a role in its formation could facilitate development of less toxic therapies. We have previously shown that E2F1 expression is deregulated in sBL. We have now investigated the mechanisms underlying E2F1 deregulation and found that the E2F sites in its promoter fail to repress its transcriptional activity in BL cells and that the transcriptional repressor E2F4 barely interacts with these sites. We also have found that E2F4 protein levels, but not those of its mRNA, are reduced in sBL cell lines relative to immortal B-cell lines. E2F4 protein expression is also decreased in 24 of 26 sBL tumor samples from patients compared with control tissues. Our data demonstrate that enforced E2F4 expression in BL cells not only diminishes E2F1 levels, but also reduces selectively the tumorigenic properties and proliferation of BL cells, while increasing their accumulation in G(2)/M. Our results therefore point to E2F4 as a target for developing novel and less toxic treatments for sBL.

Cell-context specific role of the E2F/Rb pathway in development and disease

Development of the central nervous system (CNS) requires the generation of neuronal and glial cell subtypes in appropriate numbers, and this demands the careful coordination of cell-cycle exit, survival, and differentiation. The E2F/Rb pathway is critical for cell-cycle regulation and also modulates survival and differentiation of distinct cell types in the developing and adult CNS. In this review, we first present the specific temporal patterns of expression of the E2F and Rb family members during CNS development and then discuss the genetic ablation of single or multiple members of these two families. Overall, the available data suggest a time-dependent and cell-context specific role of E2F and Rb family members in the developing and adult CNS.

Nuclear expression of E2F4 induces cell death via multiple pathways in normal human intestinal epithelial crypt cells but not in colon cancer cells

E2F transcription factors control cell cycle progression. The localization of E2F4 in intestinal epithelial cells is cell cycle dependent, being cytoplasmic in quiescent differentiated cells but nuclear in proliferative cells. However, whether nuclear translocation of E2F4 alone is sufficient to trigger intestinal epithelial cell proliferation remains to be established. Adenoviruses expressing fusion proteins between green fluorescent protein (GFP) and wild-type (wt)E2F4 or GFP and nuclear localization signal (NLS)-tagged E2F4 were used to infect normal human intestinal epithelial crypt cells (HIEC). In contrast to expression of wtE2F4, persistent expression of E2F4 into the nucleus of HIEC triggered phosphatidylserine exposure, cytoplasmic shrinkage, zeiosis, formation of apoptotic bodies, and activation of caspase 9 and caspase 3. Inhibition of caspase activities by zVAD-fmk partially inhibited cell death induced by E2F4-NLS. An induction of p53, phosphorylated Ser15-p53, PUMA, FAS, BAX, RIP, and phosphorylated JNK1 was also observed in HIEC expressing E2F4-NLS compared with wtE2F4-expressing cells. E2F1 and p14ARF expression remained unaltered. Downregulation of p53 expression by RNA interference attenuated cell death induced by E2F4-NLS. By contrast, the level of cell death was negligible in colon cancer cells despite the strong expression of E2F4 into the nucleus. In conclusion, deregulated nuclear E2F4 expression induces apoptosis via multiple pathways in normal intestinal epithelial cells but not in colon cancer cells. Hence, mutations that deregulate E2F4 localization may provide an initial proliferative advantage but at the same time accelerate cell death. However, intestinal cells acquiring mutations (e.g., p53, Bax loci, etc.) may escape apoptosis, thereby revealing the full mitogenic potential of the E2F4 transcription factor.

E2F-6 suppresses growth-associated apoptosis of human hematopoietic progenitor cells by counteracting proapoptotic activity of E2F-1

E2F-6 is a dominant-negative transcriptional repressor against other members of the E2F family. In this study, we investigated the expression and function of E2F-6 in human hematopoietic progenitor cells to clarify its role in hematopoiesis. We found that among E2F subunits, E2F-1, E2F-2, E2F-4, and E2F-6 were expressed in CD34(+) human hematopoietic progenitor cells. The expression of E2F-6 increased along with proliferation and decreased during differentiation of hematopoietic progenitors, whereas the other three species were upregulated in CD34(-) bone marrow mononuclear cells. Overexpression of E2F-6 did not affect the growth of immature hematopoietic cell line K562 but suppressed E2F-1-induced apoptosis, whereas it failed to inhibit apoptosis induced by differentiation inducers and anticancer drugs. Among E2F-1-dependent apoptosis-related molecules, E2F-6 specifically inhibited upregulation of Apaf-1 by competing with E2F-1 for promoter binding. E2F-6 similarly suppressed apoptosis and Apaf-1 upregulation in primary hematopoietic progenitor cells during cytokine-induced proliferation but had no effect when they were differentiated. As a result, E2F-6 enhanced the clonogenic growth of colony-forming unit-granulocyte, erythroid, macrophage, and megakaryocyte. These results suggest that E2F-6 provides a failsafe mechanism against loss of hematopoietic progenitor cells during proliferation. Disclosure of potential conflicts of interest is found at the end of this article.

Cardiac myocyte cell cycle control in development, disease, and regeneration

Cardiac myocytes rapidly proliferate during fetal life but exit the cell cycle soon after birth in mammals. Although the extent to which adult cardiac myocytes are capable of cell cycle reentry is controversial and species-specific differences may exist, it appears that for the vast majority of adult cardiac myocytes the predominant form of growth postnatally is an increase in cell size (hypertrophy) not number. Unfortunately, this limits the ability of the heart to restore function after any significant injury. Interest in novel regenerative therapies has led to the accumulation of much information on the mechanisms that regulate the rapid proliferation of cardiac myocytes in utero, their cell cycle exit in the perinatal period, and the permanent arrest (terminal differentiation) in adult myocytes. The recent identification of cardiac progenitor cells capable of giving rise to cardiac myocyte-like cells has challenged the dogma that the heart is a terminally differentiated organ and opened new prospects for cardiac regeneration. In this review, we summarize the current understanding of cardiomyocyte cell cycle control in normal development and disease. In addition, we also discuss the potential usefulness of cardiomyocyte self-renewal as well as feasibility of therapeutic manipulation of the cardiac myocyte cell cycle for cardiac regeneration.

Clinical relevance of E2F family members in ovarian cancer--an evaluation in a training set of 77 patients

PURPOSE

The major obstacle in treating ovarian cancer is the rapid development of platinum resistance during therapy. Deregulation of members of the E2F family of transcription factors is crucially involved in carcinogenesis and probably in mechanisms underlying platinum resistance. We therefore investigated the relevance of the whole set of E2F family members in predicting clinical outcome and their significance in predicting platinum resistance.

EXPERIMENTAL DESIGN

Real-time PCR of all E2F family members was done from 77 ovarian carcinomas, defined as our training set, and 8 healthy control samples. The correlation with clinicopathologic characteristics, platinum resistance, and survival was investigated. Furthermore, the cross-talk of E2F family members was assessed for its value in predicting survival and platinum resistance.

RESULTS

The proliferation-promoting E2F1 and E2F2 were associated with grade 3 tumors and residual disease >2 cm in diameter after initial surgery. Survival analyses showed low expression of E2F1 or E2F2 to be significantly associated with favorable disease-free and overall survival (E2F1, P = 0.039 and 0.047, respectively; E2F2, P = 0.009 and 0.006, respectively). In contrast, high expression of inhibiting E2F4 or E2F7 predicted favorable disease-free and overall survival (E2F4, P = 0.047 and 0.042, respectively; E2F7, P = 0.048 and 0.042, respectively). A high E2F2 to E2F4 ratio was the most valuable prognostic variable for disease-free survival in multivariate analysis (hazard ratio, 6.494; P = 0.002). Tumors considered platinum resistant were associated with lower E2F4 and E2F7 expression (P = 0.012 and 0.009, respectively) compared with platinum-sensitive tumors. Again, ratios of E2F1 or E2F2 to E2F7 were the most favorable variables in predicting platinum resistance.

CONCLUSIONS

We here show that deregulation of both proliferation-promoting and proliferation-inhibiting E2F transcription factors and their cross-talk is crucially involved in the tumor biology of ovarian cancer and influences clinical outcome. Furthermore, down-regulation of E2F7 may contribute to mechanisms underlying platinum resistance, and calculation of ratios of proliferation-promoting E2F1 to E2F7 could serve as a putative predictor of platinum resistance.

Methylation silencing of the Apaf-1 gene in acute leukemia

Apaf-1 is important for tumor suppression and drug resistance because it plays a central role in DNA damage-induced apoptosis. Inactivation of the Apaf-1 gene is implicated in disease progression and chemoresistance of some malignancies. In this study, we attempted to clarify the role of Apaf-1 in leukemogenesis. Apaf-1 mRNA levels were below the detection limit or very low in 5 of 20 human leukemia cell lines (25%) and 5 of 12 primary acute myeloblastic leukemia cells (42%). There were no gross structural abnormalities in the Apaf-1 gene in these samples. Expression of factors regulating Apaf-1 transcription, such as E2F-1, p53, and Sp-1, did not differ between Apaf-1-positive and Apaf-1-negative cells. Methylation of CpG in the region between +87 and +128 of the Apaf-1 gene was almost exclusively observed in Apaf-1-defective cell lines. Treatment of these cells with 5-aza-2'-deoxycytidine, a specific inhibitor of DNA methylation, restored the expression of Apaf-1. Furthermore, we showed that the region between +87 and +128 could act as a repressor element by recruiting corepressors such as methylated DNA-binding domain 2 and histone deacetylase 1 upon methylation. Overexpression of Dnmt1, a mammalian maintenance DNA methyltransferase, was associated with Apaf-1 gene methylation. DNAs from Dnmt1-overexpressing cells were more resistant to digestion with methylation-sensitive enzyme HpaII than those from cells with low Dnmt1 expression, suggesting that Dnmt1 mediates aberrant methylation of multiple genes. In conclusion, methylation silencing is a mechanism of the inactivation of Apaf-1 in acute leukemia, and Dnmt1 overexpression may underlie hypermethylation of the Apaf-1 gene.

Overlapping roles of pocket proteins in the myocardium are unmasked by germ line deletion of p130 plus heart-specific deletion of Rb

The pocket protein family of tumor suppressors, and Rb specifically, have been implicated as controlling terminal differentiation in many tissues, including the heart. To establish the biological functions of Rb in the heart and overcome the early lethality caused by germ line deletion of Rb, we used a Cre/loxP system to create conditional, heart-specific Rb-deficient mice. Mice that are deficient in Rb exclusively in cardiac myocytes (CRbL/L) are born with the expected Mendelian distribution, and the adult mice displayed no change in heart size, myocyte cell cycle distribution, myocyte apoptosis, or mechanical function. Since both Rb and p130 are expressed in the adult myocardium, we created double-knockout mice (CRbL/L p130-/-) to determine it these proteins have a shared role in regulating cardiac myocyte cell cycle progression. Adult CRbL/L p130-/- mice demonstrated a threefold increase in the heart weight-to-body weight ratio and showed increased numbers of bromodeoxyuridine- and phosphorylated histone H3-positive nuclei, consistent with persistent myocyte cycling. Likewise, the combined deletion of Rb plus p130 up-regulated myocardial expression of Myc, E2F-1, and G1 cyclin-dependent kinase activities, synergistically. Thus, Rb and p130 have overlapping functional roles in vivo to suppress cell cycle activators, including Myc, and maintain quiescence in postnatal cardiac muscle.

The nucleocytoplasmic shuttling of E2F4 is involved in the regulation of human intestinal epithelial cell proliferation and differentiation

The specific mechanisms controlling the transition from proliferation to terminal differentiation in human intestinal epithelial cells (HIEC) remain largely undefined. Herein, we analyzed the expression and localization of Rb and E2F proteins in well-established normal intestinal epithelial cell models which allow for the re-enactment of the crypt-villus axis in vitro as well as in intact epithelium and in colon cancer cells. We report that (1) expression of E2F1 is down-regulated while E2F4 protein is sequestered in the cytoplasm during G(0) arrest associated with serum deprivation, confluency, and terminal differentiation of intestinal cells; (2) concurrently, there is an accumulation of the hypophosphorylated form of the pocket proteins into the nucleus with an increased association of E2F4 with pRb and p130; (3) cells which expressed high levels of nuclear E2F4 are all positive for Ki67 staining in human fetal intestine; (4) activation of HIEC crypt cells by growth factors leads to an increase in the nuclear localization of E2F4 which may be attributable to a decrease in the serine/threonine phosphorylation of this transcription factor; (5) inhibition of p38 MAP kinase with alpha/beta inhibitor SB203580 induces E2F4 translocation into the nucleus and its transcriptional activity. In conclusion, our data suggest a key role for E2F4 in proliferation of human intestinal crypt cells and that its cytoplasmic retention as well as its sequestration by Rb proteins may represent a critical step in initiating cell-cycle exit.

Cul7/p185/p193 binding to simian virus 40 large T antigen has a role in cellular transformation

Simian virus 40 large T antigen (TAg) is a viral oncoprotein that can promote cellular transformation. TAg's transforming activity results in part by binding and inactivating key tumor suppressors, including p53 and the retinoblastoma protein (pRb). We have identified a TAg-associated 185-kDa protein that has significant homology to the cullin family of E3 ubiquitin ligases. TAg binds to an SCF-like complex that contains p185/Cul7, Rbx1, and the F box protein Fbw6. This SCF-like complex binds to an N-terminal region of TAg. Several p185/Cul7-binding-deficient mutants of TAg were generated that retained binding to pRb and p53 and were capable of overcoming Rb-mediated repression of E2F transcription. Despite binding to pRb and p53, these p185/Cul7-binding-defective mutants of TAg were unable to transform primary mouse embryo fibroblasts. Cells expressing p185/Cul7-binding-defective mutants of TAg were unable to grow to high density or grow in an anchorage-independent manner as determined by growth in soft agar. Considering the significance of other TAg-interacting proteins in regulation of the cell cycle, p185/Cul7 may also regulate an important growth control pathway.

Cyclin E and Bcl-xL cooperatively induce cell cycle progression in c-Rel−/− B cells

Aberrant overexpression of the c-rel protooncogene is associated with lymphoid malignancy, while c-rel deletion produces severe lymphoproliferative defects and immunodeficiency. To investigate the mechanism of c-rel-induced proliferation and cell cycle progression in B lymphocytes, we have compared signaling events elicited through the BCR in c-rel-/- and wild-type B cells. BCR stimulation of c-rel-/- B cells fails to induce proper cyclin expression, resulting in G1 phase arrest, but it is unclear whether these defects are in fact secondary events of decreased B-cell survival, since c-rel deletion also affects the expression of antiapoptotic genes such as bcl-xL. Here, we use the bcl-xL transgene to correct the viability of c-rel-deficient B cells, and show that the inhibition of apoptosis does not necessarily confer hyperproliferation of B cells activated through the BCR. c-rel-/- B cells still fail to enter the S phase despite improved survival by bcl-xL overexpression, suggesting that c-Rel-associated cell cycle progression is dependent on more than just enhanced cell viability. Overexpression of cyclin E protein, however, can cooperate with Bcl-xL to restore cell cycle progression to c-rel-/- B cells via induction of the cyclin-CDK/Rb-E2F pathway. Furthermore, we show that c-Rel can directly regulate transcription of the e2f3a promoter/enhancer, which is then likely to lead to transcriptional activation of the cyclin E promoter by E2F3a. Hence, these studies provide clear evidence that control of lymphocyte proliferation via c-Rel is linked to a cyclin-dependent process, and suggest that c-Rel not only activates antiapoptotic signaling but also the induction of cell cycle progression.

IFN-gamma inhibits human airway smooth muscle cell proliferation by modulating the E2F-1/Rb pathway

Elucidating the factors that inhibit the increase in airway smooth muscle (ASM) mass may be of therapeutic benefit in asthma. Here, we investigated whether interferon-gamma (IFN-gamma), a potent inducer of growth arrest in various cell types, regulates mitogen-induced ASM cell proliferation. IFN-gamma (1-100 U/ml) was found to markedly decrease both DNA synthesis and ASM cell number induced by the mitogens epidermal growth factor (EGF) and thrombin. Interestingly, IFN-gamma had no effect on mitogen-induced activation of three major mitogenic signaling pathways, phosphatidylinositol 3-kinase, p70(S6k), or mitogen-activated protein kinases. Mitogen-induced expression of cell cycle regulator cyclin D1 was increased by IFN-gamma, whereas no effect was observed on degradation of p27(Kip1). Expression array analysis of 23 cell cycle-related genes showed that IFN-gamma inhibited EGF-induced increases in E2F-1 expression, whereas induction of c-myc, cyclin D2, Egr-1, and mdm2 were unaffected. Induction of E2F-1 protein and Rb hyperphosphorylation after mitogen stimulation was also suppressed by IFN-gamma. In addition, IFN-gamma decreased activation of cdk2 and expression of cyclin E, upstream signaling molecules responsible for Rb hyperphosphorylation in the late G1 phase. IFN-gamma also increased levels of IFI 16 protein, whose mouse homolog p202 has been associated with growth inhibition. Together, our data indicate that IFN-gamma is an effective inhibitor of ASM cell proliferation by blocking transition from G1-to-S phase by acting at two different levels: modulation of cdk2/cyclin E activation and inhibition of E2F-1 gene expression.

Potential of interferon-alpha in solid tumours: part 2

The second part of this review examines the use of recombinant interferon-alpha (rIFNalpha) in the following solid tumours: superficial bladder cancer, Kaposi's sarcoma, head and neck cancer, gastrointestinal cancers, lung cancer, mesothelioma and ovarian, breast and cervical malignancies. In superficial bladder cancer, intravesical rIFNalpha has a promising role as second-line therapy in patients resistant or intolerant to intravesical bacille Calmette-Guérin (BCG). In HIV-associated Kaposi's sarcoma, rIFNalpha is active as monotherapy and in combination with antiretroviral agents, especially in patients with CD4 counts >200/mm(3), no prior opportunistic infections and nonvisceral disease. rIFNalpha has shown encouraging results when used in combination with retinoids in the chemoprevention of head and neck squamous cell cancers. It is effective in the chemoprevention of hepatocellular cancer in hepatitis C-seropositive patients. In neuroendocrine tumours, including carcinoid tumour, low-dosage (</=3 MU) or intermediate-dosage (5 to 10 MU) rIFNalpha is indicated as second-line treatment, either with octreotide or alone in patients resistant to somatostatin analogues. Intracavitary IFNalpha may be useful in malignant pleural effusions from mesothelioma. Similarly, intraperitoneal IFNalpha may have a role in the treatment of minimal residual disease in ovarian cancer. In breast cancer, the only possible role for IFNalpha appears to be intralesional administration for resistant disease. IFNalpha may have a role as a radiosensitising agent for the treatment of cervical cancer; however, this requires confirmation in randomised trials. On the basis of current evidence, the routine use of rIFNalpha is not recommended in the therapy of head and neck squamous cell cancers, upper gastrointestinal tract, colorectal and lung cancers, or mesothelioma. Pegylated IFNalpha (peginterferon-alpha) is an exciting development that offers theoretical advantages of increased efficacy, reduced toxicity and improved compliance. Further data from randomised studies in solid tumours are needed where rIFNalpha has activity, such as neuroendocrine tumours, minimal residual disease in ovarian cancer, and cervical cancer. A better understanding of the biological mechanisms that determine response to rIFNalpha is needed. Studies of IFNalpha-stimulated gene expression, which are now feasible, should help to identify molecular predictors of response and allow us to target therapy more selectively to patients with solid tumours responsive to IFNalpha.

N-Acetylcysteine enhances UV-mediated caspase-3 activation, fragmentation of E2F-4, and apoptosis in human C8161 melanoma: inhibition by ectopic Bcl-2 expression

Redox imbalance due to oxidative stress or excessive antioxidant levels can alter apoptotic responses. Recently, antioxidants like N-acetylcysteine (NAC) were reported to inhibit H(2)O(2)-mediated necrotic cell death, although they were inactive against apoptosis induced by other agents like etoposide. NAC was also found to kill preferentially tumor cells compared to normal fibroblasts at 20-50mM, but these concentrations are lethal to normal splenocytes. We now demonstrate that 10mM NAC, a non-toxic concentration, can enhance the UV radiation-mediated apoptosis of human C8161 melanoma cells. Compared to treatment with UV radiation alone, combination treatment with NAC doubled the ratio of activated caspase-3 to pro-caspase-3 and produced greater fragmentation of the retinoblastoma protein and the E2F-4 transcription factor without affecting the E2F-1 protein. These effects of joint NAC-UV radiation treatment were counteracted by the overexpression of the bcl-2 gene. To our knowledge, this report is the first to: (i) demonstrate a synergy between DNA-damaging agents, like UV radiation, and antioxidants, like NAC, and (ii) show that a Bcl-2-inhibitable E2F-4 fragmentation occurs concurrently with caspase-3 activation and apoptosis.

An E2F-binding site mediates the activation of the proliferative isoform of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by phosphatidylinositol 3-kinase

In the present study, we demonstrate that E2F is implicated in the regulation of the glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/Fru-2,6-BPase) during cell division. The expression of this enzyme is induced during the G(1)/S transition of the cell cycle. We identified and monitored the E2F-pocket protein complexes that bind to the E2F site of the F-type promoter during cell-cycle entry, and we analysed their contribution to the phosphatidylinositol 3-kinase (PI 3-kinase)-mediated regulation of the promoter. We found that the predominant E2F complex bound to the F-type promoter in unstimulated/quiescent cells contains E2F4, DP1 and p130 proteins. In serum-stimulated (S-phase) cells, the composition of the complex switched to E2F1/4, DP1 and p107, together with cyclin A and cyclin-dependent kinase 2. Treatment with the PI 3-kinase specific inhibitor LY 294002 prevented the formation of the S-phase complex, suggesting that activation of the PI 3-kinase pathway is essential for the formation of this complex. Further supporting this idea, we obtained results showing that treatment of cycling NIH 3T3 cells with either wortmannin or LY 294002 induces the accumulation of the transcriptionally repressive p130-E2F4-DP1 complex. Using the Rat-1 ER-E2F1 cell line where E2F1 activity can be conditionally induced, we demonstrated that E2F activity is involved in the in vivo transcriptional regulation of the F-type 6PF2K/Fru-2, 6-BPase gene. Taken together, our results show that the F-type 6PF2K/Fru-2, 6-BPase is a genuine E2F-regulated gene, and that its regulation by the PI 3-kinase pathway is at least partially mediated through the E2F transcription factor.

The pRb-related protein p130 is regulated by phosphorylation-dependent proteolysis via the protein-ubiquitin ligase SCF(Skp2)

p130 is a tumor suppressor of the pocket protein family whose expression is posttranscriptionally regulated and largely G0 restricted. The mechanism of down-regulation of p130 expression in proliferating cells was investigated. Our results indicate that the decline of p130 expression as G0 cells reenter the cell cycle is due to a decrease in protein stability. The enhancement of p130 turnover in late G1 and S phase compared with G0 and early G1 phase was dependent on Cdk4/6-specific phosphorylation of p130 on Serine 672, and independent of Cdk2 activity. The activity of the ubiquitin ligase complex Skp1-Cul1/Cdc53-F-box protein Skp2 (SCF(Skp2)) and the proteasome were necessary for p130 degradation. In vitro, recombinant Skp2 was able to bind hyperphosphorylated but not dephosphorylated p130. Furthermore, in vitro polyubiquitination of p130 by SCF(Skp2) was specifically dependent on phosphorylation of p130 on Serine 672. Thus, like the Cdk inhibitor p27(Kip1), p130 turnover is regulated by Cdk-dependent G1 phosphorylation leading to ubiquitin-dependent proteolysis.

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

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

A B-myb promoter corepressor site facilitates in vivo occupation of the adjacent E2F site by p107 x E2F and p130 x E2F complexes

Transcription from the B-myb (MybL2 gene) promoter is strictly cell cycle-regulated by repression mediated through an E2F site during G(0)/early G(1). We report here the characterization of a corepressor site (downstream repression site (DRS)) required for this activity that is closely linked to the E2F site. Systematic mutagenesis of the DRS enabled a consensus to be derived, and it is notable that this sequence is compatible with cell cycle gene homology region sequences associated with cell cycle-dependent elements in the cyclin A, cdc2, and CDC25C promoters. The B-myb promoter is inappropriately active during G(0) in mouse embryo fibroblasts lacking the p107 and p130 pocket proteins, and we show that the ability of transfected p107 and p130 to re-impose repression on the promoter is dependent on the DRS. In contrast, transfected Rb was unable to repress the B-myb promoter. Consistent with the notion that Rb.E2F complexes are unable to bind the B-myb promoter E2F site in vivo, footprinting showed that this site is unoccupied in cells lacking p107 and p130. Chromatin immunoprecipitation assays showed a requirement for the DRS in recruiting p107 and p130 complexes to the B-myb promoter, indicating that in vivo the DRS governs the occupancy of the adjacent E2F site by transcriptional repressors.

Review: IFN-alpha/beta receptor interactions to biologic outcomes: understanding the circuitry

Type I interferons (IFNs), which include the IFN-alphas, IFN-beta, IFN-omega, IFN-kappa, and IFN-tau, are an evolutionarily conserved group of secreted cytokines that serve as potent extracellular mediators of host defense and homeostasis. Binding of IFNs to specific cell surface receptors results in the activation of multiple intracellular signaling cascades, leadingto the synthesis of proteins that mediate antiviral, growth inhibitory and immunomodulatory responses. In the past decade, considerable information has accumulated pertaining to the different signalingpathways that are activated by the type I IFNs. Although many of the literature findings are specific to defined cell systems or are tissue restricted, the intent of this review is to place these signaling cascades and their effectors in the context of distinct biologic outcomes.

Differences in E2F subunit expression in quiescent and proliferating vascular smooth muscle cells

E2F is a family of transcriptional factors that control G(1)/S transition. We investigated how the E2F family participates in the biological responses of vascular smooth muscle cells (VSMC) to vasoconstrictive hormones compared with fetal bovine serum (FBS). FBS induced upregulation of E2F-1 and E2F-5 at both mRNA and protein levels and slightly reduced E2F-3 protein. Angiotensin II (ANG II) and arginine vasopressin increased E2F-3 protein, but not E2F-1 and E2F-5, without upregulating its mRNA level. FBS transactivated the E2F-1 gene through the induction of free E2F-1 binding onto its promoter, whereas ANG II-induced binding of E2F-3 did not result in activation of the E2F-1 promoter. These changes are responsible for hypertrophic or hyperplastic response of VSMC to different growth factors or stimulants. In contrast, both FBS and vasoconstrictive hormones drove transcription of the cdc6 gene by downregulating p130 and recruiting free E2F-3 in the latter, which underlies the progression of VSMC into S phase.

Control of cell cycle exit and entry by protein kinase B-regulated forkhead transcription factors

AFX-like Forkhead transcription factors, which are controlled by phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) signaling, are involved in regulating cell cycle progression and cell death. Both cell cycle arrest and induction of apoptosis are mediated in part by transcriptional regulation of p27(kip1). Here we show that the Forkheads AFX (FOXO4) and FKHR-L1 (FOXO3a) also directly control transcription of the retinoblastoma-like p130 protein and cause upregulation of p130 protein expression. Detailed analysis of p130 regulation demonstrates that following Forkhead-induced cell cycle arrest, cells enter G(0) and become quiescent. This is shown by a change in phosphorylation of p130 to G(0)-specific forms and increased p130/E2F-4 complex formation. Most importantly, long-term Forkhead activation causes a sustained but reversible inhibition of proliferation without a marked increase in apoptosis. As for the activity of the Forkheads, we also show that protein levels of p130 are controlled by endogenous PI3K/PKB signaling upon cell cycle reentry. Surprisingly, not only nontransformed cells, but also cancer cells such as human colon carcinoma cells, are forced into quiescence by Forkhead activation. We therefore propose that Forkhead inactivation by PKB signaling in quiescent cells is a crucial step in cell cycle reentry and contributes to the processes of transformation and regeneration.

Phosphorylation-dependent and -independent functions of p130 cooperate to evoke a sustained G1 block

The retinoblastoma (pRb)-related p130 pocket protein is a regulator of cell growth and differentiation, and a candidate tumour suppressor. Both pRb and p130 operate through interactions with cellular proteins, including the E2F transcription factors. While such interactions are controlled by phosphorylation of multiple sites of pRb, regulation of p130 remains poorly understood. We now identify 22 in vivo phosphorylation sites of p130, targeted by diverse kinases, and present evidence for three cyclin-dependent kinase 4(6) [Cdk4(6)] specific phosphorylations, which appear critical for controlling the growth-restraining activity of p130. When expressed in U2OS cells, the phosphorylation-deficient mutant p130(Delta)(CDK4), in which the Cdk4 specific sites were mutated to alanine residues, imposed a more sustained G1 arrest than a constitutively active pRb(Delta)(CDK), known to repress all cellular E2F activity. Experiments using p130(Delta)(Cdk4) and another phosphorylation-deficient mutant, p130(PM19A), with 19 phosphorylation sites mutated, revealed that the p130-imposed G1 block reflects cooperative growth-suppressive effects of phosphorylation-regulated E2F binding and phosphorylation-independent sequestration of cyclin E(A)-Cdk2 through the N-terminal cyclin binding motif of p130.

Mullerian inhibiting substance inhibits ovarian cell growth through an Rb-independent mechanism

Müllerian inhibiting substance (MIS), a transforming growth factor-beta family member, causes regression of the Müllerian duct in male embryos. MIS overexpression in transgenic mice ablates the ovary, and MIS inhibits the growth of ovarian cancer cell lines in vitro, suggesting a key role for this hormone in postnatal development of the ovary. This report describes a mechanism for MIS-mediated growth inhibition in both a human epithelial ovarian cancer cell line and a cell line derived from normal ovarian surface epithelium, which is the origin of human epithelial ovarian cancers. MIS-treated cells accumulated in the G(1) phase of the cell cycle and subsequently underwent apoptosis. MIS up-regulated the cyclin-dependent kinase inhibitor p16 through an MIS type II receptor-mediated mechanism and inhibited growth in the absence of detectable or inactive Rb protein. Prolonged treatment with MIS down-regulated the Rb-related protein p130 and increased the Rb family-regulated transcription factor E2F1, overexpression of which inhibited growth. These findings demonstrate that p16 is required for MIS-mediated growth inhibition in ovarian epithelial cells and tumor cells and suggest that up-regulation of E2F1 also plays a role in this process.

Phosphorylation of the retinoblastoma-related protein p130 in growth-arrested cells

The retinoblastoma family of proteins including pRB, p107 and p130 undergoes cell cycle dependent phosphorylation during the mid-G1 to S phase transition. This phosphorylation is dependent upon the activity of cyclin D/cdk4. In contrast to pRB and p107, p130 is phosphorylated during G0 and the early G1 phase of the cell cycle. We observed that p130 is specifically phosphorylated on serine and threonine residues in T98G cells arrested in G0 by serum deprivation or density arrest. Identification of the phospho-serine and phospho-threonine residues revealed that most were clustered within a short co-linear region unique to p130, defined as the Loop. Deletion of the Loop region resulted in a change in the phosphorylation status of p130 under growth arrest conditions. Notably, deletion of the Loop did not affect the ability of p130 to bind to E2F-4 or SV40 Large T antigen, to induce growth arrest in Saos-2 cells, and to become hyperphosphorylated during the proliferative phase of the cell cycle. p130 undergoes specific G0 phosphorylation in a manner that distinguishes it from pRB and p107.

Bcl-2 retards cell cycle entry through p27(Kip1), pRB relative p130, and altered E2F regulation

Independent of its antiapoptotic function, Bcl-2 can, through an undetermined mechanism, retard entry into the cell cycle. Cell cycle progression requires the phosphorylation by cyclin-dependent kinases (Cdks) of retinoblastoma protein (pRB) family members to free E2F transcription factors. We have explored whether retarded cycle entry is mediated by the Cdk inhibitor p27 or the pRB family. In quiescent fibroblasts, enforced Bcl-2 expression elevated levels of both p27 and the pRB relative p130. Bcl-2 still slowed G(1) progression in cells deficient in pRB but not in those lacking p27 or p130. Hence, pRB is not required, but both p27 and p130 are essential mediators. The ability of p130 to form repressive complexes with E2F4 is implicated, because the retardation by Bcl-2 was accentuated by coexpressed E2F4. A plausible relevant target of p130/E2F4 is the E2F1 gene, because Bcl-2 expression delayed E2F1 accumulation during G(1) progression and overexpression of E2F1 overrode the Bcl-2 inhibition. Hence, Bcl-2 appears to retard cell cycle entry by increasing p27 and p130 levels and maintaining repressive complexes of p130 with E2F4, perhaps to delay E2F1 expression.

Deregulated E2F transcriptional activity in autonomously growing melanoma cells

Inactivation of the retinoblastoma tumor suppressor protein (pRb) has been implicated in melanoma cells, but the molecular basis for this phenotype has not yet been elucidated, and the status of additional family members (p107 and p130, together termed pocket proteins) or the consequences on downstream targets such as E2F transcription factors are not known. Because cell cycle progression is dependent on the transcriptional activity of E2F family members (E2F1-E2F6), most of them regulated by suppressive association with pocket proteins, we characterized E2F-pocket protein DNA binding activity in normal versus malignant human melanocytes. By gel shift analysis, we show that in mitogen-dependent normal melanocytes, external growth factors tightly controlled the levels of growth-promoting free E2F DNA binding activity, composed largely of E2F2 and E2F4, and the growth-suppressive E2F4-p130 complexes. In contrast, in melanoma cells, free E2F DNA binding activity (E2F2 and E2F4, to a lesser extent E2F1, E2F3, and occasionally E2F5), was constitutively maintained at high levels independently of external melanocyte mitogens. E2F1 was the only family member more abundant in the melanoma cells compared with normal melanocytes, and the approximately fivefold increase in DNA binding activity could be accounted for mostly by a similar increase in the levels of the dimerization partner DP1. The continuous high expression of cyclin D1, A2, and E, the persistent cyclin-dependent kinase 4 (CDK4) and CDK2 activities, and the presence of hyperphosphorylated forms of pRb, p107, and p130, suggest that melanoma cells acquired the capacity for autonomous growth through inactivation of all three pocket proteins and release of E2F activity, otherwise tightly regulated in normal melanocytes by external growth factors.

Activation of a cAMP pathway and induction of melanogenesis correlate with association of p16(INK4) and p27(KIP1) to CDKs, loss of E2F-binding activity, and premature senescence of human melanocytes

There is strong evidence that the senescent phenotype, whether induced by telomere shortening, oxidative damage, or oncogenic stimuli, is an important tumor suppressive mechanism. The melanocyte is a cell of neural crest origin that produces the pigment melanin and can develop into malignant melanomas. To understand how malignant cells escape senescence, it is first crucial to define what genes control senescence in the normal cell. Prolonged exposure to high levels of cAMP results in accumulation of melanin and terminal differentiation of human melanocytes. Here we present evidence that activation of a cAMP pathway correlates with multiple cellular changes in these cells: (1) increased expression of the transcription factor microphthalmia; (2) increased melanogenesis; (3) increased association of the cyclin-dependent kinase inhibitors (CDK-Is) p27(KIP1) and p16(INK4) with CDK2 and CDK4, respectively; (4) failure to phosphorylate the retinoblastoma protein (pRB); (5) decreased expression of E2F1, E2F2, and E2F4 proteins; (6) loss of E2F DNA-binding activity; and (7) phenotypic changes characteristic of senescent cells. Senescent melanocytes have potent E2F inhibitory activity, because extracts from these cells completely abolished E2F DNA-binding activity that was present in extracts from the early proliferative phase. We propose that increased activity of the CDK-Is p27 and p16 and loss of E2F activity in human melanocytes characterize a senescence program activated by the cAMP pathway. Disruption of cAMP-mediated and melanogenesis-induced senescence may cause immortalization of human melanocytes, an early step in the development of melanomas.