Human ARF binds E2F1 and inhibits its transcriptional activity

Molecular mechanisms underlying the regulation of tumour suppressor genes in lung cancer

Tumour suppressor genes play a cardinal role in the development of a large array of human cancers, including lung cancer, which is one of the most frequently diagnosed cancers worldwide. Therefore, extensive studies have been committed to deciphering the underlying mechanisms of alterations of tumour suppressor genes in governing tumourigenesis, as well as resistance to cancer therapies. In spite of the encouraging clinical outcomes demonstrated by lung cancer patients on initial treatment, the subsequent unresponsiveness to first-line treatments manifested by virtually all the patients is inherently a contentious issue. In light of the aforementioned concerns, this review compiles the current knowledge on the molecular mechanisms of some of the tumour suppressor genes implicated in lung cancer that are either frequently mutated and/or are located on the chromosomal arms having high LOH rates (1p, 3p, 9p, 10q, 13q, and 17p). Our study identifies specific genomic loci prone to LOH, revealing a recurrent pattern in lung cancer cases. These loci, including 3p14.2 (FHIT), 9p21.3 (p16INK4a), 10q23 (PTEN), 17p13 (TP53), exhibit a higher susceptibility to LOH due to environmental factors such as exposure to DNA-damaging agents (carcinogens in cigarette smoke) and genetic factors such as chromosomal instability, genetic mutations, DNA replication errors, and genetic predisposition. Furthermore, this review summarizes the current treatment landscape and advancements for lung cancers, including the challenges and endeavours to overcome it. This review envisages inspired researchers to embark on a journey of discovery to add to the list of what was known in hopes of prompting the development of effective therapeutic strategies for lung cancer.

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

p16 INK4A drives nonalcoholic fatty liver disease phenotypes in high fat diet fed mice through biliary E2F1/FOXO1/IGF-1 signaling

BACKGROUND AND AIMS

NAFLD is characterized by steatosis, hepatic inflammation, and fibrosis, which can develop into NASH. Patients with NAFLD/NASH have increased ductular reaction (DR) and biliary senescence. High fat/high cholesterol diet feeding increases biliary senescence, DR, and biliary insulin-like growth factor-1 (IGF-1) expression in mice. p16/IGF-1 converges with fork-head box transcription factor O1 (FOXO1) through E2F1. We evaluated p16 inhibition on NAFLD phenotypes and biliary E2F1/FOXO1/IGF-1 signaling.

APPROACH AND RESULTS

4-week wild-type (C57BL/6J) male mice were fed a control diet (CD) or high fat/high cholesterol diet and received either p16 or control Vivo Morpholino (VM) by tail vein injection 2× during the 16th week of feeding. We confirmed p16 knockdown and examined: (i) NAFLD phenotypes; (ii) DR and biliary senescence; (iii) serum metabolites; and (iv) biliary E2F1/FOXO1/IGF-1 signaling. Human normal, NAFLD, and NASH liver samples and isolated cholangiocytes treated with control or p16 VM were evaluated for p16/E2F1/FOXO1/IGF-1 signaling. p16 VM treatment reduced cholangiocyte and hepatocyte p16. In wild-type high fat/high cholesterol diet mice with control VM, there were increased (i) NAFLD phenotypes; (ii) DR and biliary senescence; (iii) serum metabolites; and (iv) biliary E2F1/FOXO1/IGF-1 signaling; however, p16 VM treatment reduced these parameters. Biliary E2F1/FOX-O1/IGF-1 signaling increased in human NAFLD/NASH but was blocked by p16 VM. In vitro , p16 VM reduced biliary E2f1 and Foxo1 transcription by inhibiting RNA pol II binding and E2F1 binding at the Foxo1 locus, respectively. Inhibition of E2F1 reduced biliary FOXO1 in vitro.

CONCLUSION

Attenuating hepatic p16 expression may be a therapeutic approach for improving NAFLD/NASH phenotypes.

It’s Getting Complicated—A Fresh Look at p53-MDM2-ARF Triangle in Tumorigenesis and Cancer Therapy

Anti-tumorigenic mechanisms mediated by the tumor suppressor p53, upon oncogenic stresses, are our bodies’ greatest weapons to battle against cancer onset and development. Consequently, factors that possess significant p53-regulating activities have been subjects of serious interest from the cancer research community. Among them, MDM2 and ARF are considered the most influential p53 regulators due to their abilities to inhibit and activate p53 functions, respectively. MDM2 inhibits p53 by promoting ubiquitination and proteasome-mediated degradation of p53, while ARF activates p53 by physically interacting with MDM2 to block its access to p53. This conventional understanding of p53-MDM2-ARF functional triangle have guided the direction of p53 research, as well as the development of p53-based therapeutic strategies for the last 30 years. Our increasing knowledge of this triangle during this time, especially through identification of p53-independent functions of MDM2 and ARF, have uncovered many under-appreciated molecular mechanisms connecting these three proteins. Through recognizing both antagonizing and synergizing relationships among them, our consideration for harnessing these relationships to develop effective cancer therapies needs an update accordingly. In this review, we will re-visit the conventional wisdom regarding p53-MDM2-ARF tumor-regulating mechanisms, highlight impactful studies contributing to the modern look of their relationships, and summarize ongoing efforts to target this pathway for effective cancer treatments. A refreshed appreciation of p53-MDM2-ARF network can bring innovative approaches to develop new generations of genetically-informed and clinically-effective cancer therapies.

Upregulation of 5'-terminal oligopyrimidine mRNA translation upon loss of the ARF tumor suppressor

Tumor cells require nominal increases in protein synthesis in order to maintain high proliferation rates. As such, tumor cells must acquire enhanced ribosome production. How the numerous mutations in tumor cells ultimately achieve this aberrant production is largely unknown. The gene encoding ARF is the most commonly deleted gene in human cancer. ARF plays a significant role in regulating ribosomal RNA synthesis and processing, ribosome export into the cytoplasm, and global protein synthesis. Utilizing ribosome profiling, we show that ARF is a major suppressor of 5'-terminal oligopyrimidine mRNA translation. Genes with increased translational efficiency following loss of ARF include many ribosomal proteins and translation factors. Knockout of p53 largely phenocopies ARF loss, with increased protein synthesis and expression of 5'-TOP encoded proteins. The 5'-TOP regulators eIF4G1 and LARP1 are upregulated in Arf- and p53-null cells.

Post-Translational Regulation of ARF: Perspective in Cancer

Tumorigenesis can be induced by various stresses that cause aberrant DNA mutations and unhindered cell proliferation. Under such conditions, normal cells autonomously induce defense mechanisms, thereby stimulating tumor suppressor activation. ARF, encoded by the CDKN2a locus, is one of the most frequently mutated or deleted tumor suppressors in human cancer. The safeguard roles of ARF in tumorigenesis are mainly mediated via the MDM2-p53 axis, which plays a prominent role in tumor suppression. Under normal conditions, low p53 expression is stringently regulated by its target gene, MDM2 E3 ligase, which induces p53 degradation in a ubiquitin-proteasome-dependent manner. Oncogenic signals induced by MYC, RAS, and E2Fs trap MDM2 in the inhibited state by inducing ARF expression as a safeguard measure, thereby activating the tumor-suppressive function of p53. In addition to the MDM2-p53 axis, ARF can also interact with diverse proteins and regulate various cellular functions, such as cellular senescence, apoptosis, and anoikis, in a p53-independent manner. As the evidence indicating ARF as a key tumor suppressor has been accumulated, there is growing evidence that ARF is sophisticatedly fine-tuned by the diverse factors through transcriptional and post-translational regulatory mechanisms. In this review, we mainly focused on how cancer cells employ transcriptional and post-translational regulatory mechanisms to manipulate ARF activities to circumvent the tumor-suppressive function of ARF. We further discussed the clinical implications of ARF in human cancer.

Regulatory Network of ARF in Cancer Development

ARF is a tumor suppressor protein that has a pivotal role in the prevention of cancer development through regulating cell proliferation, senescence, and apoptosis. As a factor that induces senescence, the role of ARF as a tumor suppressor is closely linked to the p53-MDM2 axis, which is a key process that restrains tumor formation. Thus, many cancer cells either lack a functional ARF or p53, which enables them to evade cell oncogenic stress-mediated cycle arrest, senescence, or apoptosis. In particular, the ARF gene is a frequent target of genetic and epigenetic alterations including promoter hyper-methylation or gene deletion. However, as many cancer cells still express ARF, pathways that negatively modulate transcriptional or post-translational regulation of ARF could be potentially important means for cancer cells to induce cellular proliferation. These recent findings of regulators affecting ARF protein stability along with its low levels in numerous human cancers indicate the significance of an ARF post-translational mechanism in cancers. Novel findings of regulators stimulating or suppressing ARF function would provide new therapeutic targets to manage cancer- and senescence-related diseases. In this review, we present the current knowledge on the regulation and alterations of ARF expression in human cancers, and indicate the importance of regulators of ARF as a prognostic marker and in potential therapeutic strategies.

Clinicopathological significance of p14ARF expression in lung cancer: a meta-analysis

BACKGROUND

p14ARF, a tumor suppressor protein, encoded by the p16 tumor suppressor gene, has been reported to be associated with the clinicopathological features of lung cancer. However, the evaluated outcomes were inconsistent and remained inconclusive. In this study, we conducted a meta-analysis to clarify the significance of p14ARF expression in lung cancer pathogenesis.

MATERIALS AND METHODS

Electronic databases, PubMed, Web of Knowledge, Embase, and CNKI, were retrieved to collect relevant articles with inclusion and exclusion criteria. Using Stata 12.0 software, 95% confidence intervals (CIs) and odds ratios (ORs) were calculated.

RESULTS

A total of 15 eligible case-control studies that evaluated the relationship between p14ARF expression and lung cancer were included in the meta-analysis. The results demonstrated that there were significant associations between p14ARF expression and the risk of non-small-cell lung cancer (NSCLC), lung adenocarcinoma, and lung squamous carcinoma (for NSCLC, OR =11.02, 95% CI =5.30-22.92; for lung adenocarcinoma, OR =7.28, 95% CI =3.92-13.50; and for lung squamous carcinoma, OR =14.40, 95% CI =2.83-73.24). In the stratified analysis based on race, significant associations between p14ARF expression and lung cancer risk were found in Chinese population and Caucasians (for Chinese population, OR = 7.02, 95% CI =4.48-11.00 and for Caucasians, OR =4.19, 95% CI =1.42-12.38). Furthermore, the expression of p14ARF was significantly associated with the TNM-stage of lung cancer in Chinese population (OR =2.07, 95% CI =1.38-3.10).

CONCLUSION

p14ARF expression was significantly associated with the risk of lung cancer. In addition, the data of the meta-analysis showed that there was a significant correlation between p14ARF expression and the TNM-stage of lung cancer in Chinese population.

SENP8 limits aberrant neddylation of NEDD8 pathway components to promote cullin-RING ubiquitin ligase function

NEDD8 is a ubiquitin-like modifier most well-studied for its role in activating the largest family of ubiquitin E3 ligases, the cullin-RING ligases (CRLs). While many non-cullin neddylation substrates have been proposed over the years, validation of true NEDD8 targets has been challenging, as overexpression of exogenous NEDD8 can trigger NEDD8 conjugation through the ubiquitylation machinery. Here, we developed a deconjugation-resistant form of NEDD8 to stabilize the neddylated form of cullins and other non-cullin substrates. Using this strategy, we identified Ubc12, a NEDD8-specific E2 conjugating enzyme, as a substrate for auto-neddylation. Furthermore, we characterized SENP8/DEN1 as the protease that counteracts Ubc12 auto-neddylation, and observed aberrant neddylation of Ubc12 and other NEDD8 conjugation pathway components in SENP8-deficient cells. Importantly, loss of SENP8 function contributes to accumulation of CRL substrates and defective cell cycle progression. Thus, our study highlights the importance of SENP8 in maintaining proper neddylation levels for CRL-dependent proteostasis.

Dynamics of ARF regulation that control senescence and cancer

ARF is an alternative reading frame product of the INK4a/ARF locus, inactivated in numerous human cancers. ARF is a key regulator of cellular senescence, an irreversible cell growth arrest that suppresses tumor cell growth. It functions by sequestering MDM2 (a p53 E3 ligase) in the nucleolus, thus activating p53. Besides MDM2, ARF has numerous other interacting partners that induce either cellular senescence or apoptosis in a p53-independent manner. This further complicates the dynamics of the ARF network. Expression of ARF is frequently disrupted in human cancers, mainly due to epigenetic and transcriptional regulation. Vigorous studies on various transcription factors that either positively or negatively regulate ARF transcription have been carried out. However, recent focus on posttranslational modifications, particularly ubiquitination, indicates wider dynamic controls of ARF than previously known. In this review, we discuss the role and dynamic regulation of ARF in senescence and cancer.

p19(Arf) limits primary vitreous cell proliferation driven by PDGF-B

Arf encodes an important tumor suppressor, p19(Arf), which also plays a critical role to control hyperplasia in the primary vitreous during mouse eye development. In the absence of Arf, mice are born blind and display a phenotype closely mimicking severe forms of the human eye disease, persistent hyperplastic primary vitreous (PHPV). In this report, we characterize p19(Arf) expression in perivascular cells that normally populate the primary vitreous and express the Arf promoter. Using a new ex vivo model, we show that these cells respond to exogenous Tgfβ, despite being isolated at a time when Tgfβ has already turned on the Arf promoter. Treatment of the cells with PDGF-B ligand doubles the population of cells in S-phase and ectopic expression of Arf blunts that effect. We show this effect is mediated through Pdgfrβ as expression of Arf represses expression of Pdgfrβ mRNA and protein to approximately 60%. p53 is not required for Arf-dependent blockade of PDGF-B driven proliferation and repression of Pdgfrβ protein as ectopic expression of Arf is still able to inhibit the 2-fold increase in the S-phase fraction of cells upon treatment with PDGF-B. Finally, induction of mature miR-34a, a microRNA previously identified to be regulated by p19(Arf) does not depend on p53 while the expression of the primary transcript does require p53. These data corroborate that, as in vivo, p19(Arf) functions to inhibit PDGF-B driven proliferation ex vivo.

Post-translational Down-regulation of Melanoma Antigen-A11 (MAGE-A11) by Human p14-ARF Tumor Suppressor

X-linked primate-specific melanoma antigen-A11 (MAGE-A11) is a human androgen receptor (AR) coactivator and proto-oncogene expressed at low levels in normal human reproductive tract tissues and at higher levels in castration-resistant prostate cancer where it is required for androgen-dependent cell growth. In this report, we show that MAGE-A11 is targeted for degradation by human p14-ARF, a tumor suppressor expressed from an alternative reading frame of the p16 cyclin-dependent kinase inhibitor INK4a/ARF gene. MAGE-A11 degradation by the proteasome was mediated by an interaction with p14-ARF and was independent of lysine ubiquitination. A dose-dependent inverse relationship between MAGE-A11 and p14-ARF correlated with p14-ARF inhibition of the MAGE-A11-induced increase in androgen-dependent AR transcriptional activity and constitutive activity of a splice variant-like AR. Reciprocal stabilization between MAGE-A11 and AR did not protect against degradation promoted by p14-ARF. p14-ARF prevented MAGE-A11 interaction with the E2F1 oncoprotein and inhibited the MAGE-A11-induced increase in E2F1 transcriptional activity. Post-translational down-regulation of MAGE-A11 promoted by p14-ARF was independent of HDM2, the human homologue of mouse double minute 2, an E3 ubiquitin ligase inhibited by p14-ARF. However, MAGE-A11 had a stabilizing effect on HDM2 in the absence or presence of p14-ARF and cooperated with HDM2 to increase E2F1 transcriptional activity in the absence of p14-ARF. We conclude that degradation of MAGE-A11 promoted by the human p14-ARF tumor suppressor contributes to low levels of MAGE-A11 in nontransformed cells and that higher levels of MAGE-A11 associated with low p14-ARF increase AR and E2F1 transcriptional activity and promote the development of castration-resistant prostate cancer.

Tumor suppressor ARF: The new player of innate immunity

ARF (alternative reading frame) is one of the most important tumor regulator playing critical roles in controlling tumor initiation and progression. Recently, we have demonstrated a novel and unexpected role for ARF as modulator of inflammatory responses.

Isolation and characterization of mammalian cells expressing the Arf promoter during eye development

Although many researchers have successfully uncovered novel functions of the tumor suppressor p19(Arf) utilizing various types of cultured cancer cells and immortalized fibroblasts, these systems do not accurately reflect the endogenous environment in which Arf is developmentally expressed. We addressed this by isolating perivascular cells (PVCs) from the primary vitreous of the mouse eye. This rare cell type normally expresses the p19(Arf) tumor suppressor in a non-pathological, developmental context. We utilized fluorescence activated cell sorting (FACS) to purify the cells by virtue of a GFP reporter driven by the native Arf promoter and then characterized their morphology and gene expression pattern. We further examined the effects of reintroduction of Arf expression in the Arf(GFP/GFP) PVCs to verify expected downstream effectors of p19(Arf) as well as uncover novel functions of Arf as a regulator of vasculogenesis. This methodology and cell culture model should serve as a useful tool to examine p19(Arf) biology.

Characterization of the human cumulus cell transcriptome during final follicular maturation and ovulation

Cumulus expansion and oocyte maturation are central processes in ovulation. Knowledge gained from rodent and other mammalian models has revealed some of the molecular pathways associated with these processes. However, the equivalent pathways in humans have not been thoroughly studied and remain unidentified. Compact cumulus cells (CCs) from germinal vesicle cumulus oocyte complexes (COCs) were obtained from patients undergoing in vitro maturation (IVM) procedures. Expanded CCs from metaphase 2 COC were obtained from patients undergoing IVF/ICSI. Global transcriptome profiles of the samples were obtained using state-of-the-art RNA sequencing techniques. We identified 1746 differentially expressed (DE) genes between compact and expanded CCs. Most of these genes were involved in cellular growth and proliferation, cellular movement, cell cycle, cell-to-cell signaling and interaction, extracellular matrix and steroidogenesis. Out of the DE genes, we found 89 long noncoding RNAs, of which 12 are encoded within introns of genes known to be involved in granulosa cell processes. This suggests that unique noncoding RNA transcripts may contribute to the regulation of cumulus expansion and oocyte maturation. Using global transcriptome sequencing, we were able to generate a library of genes regulated during cumulus expansion and oocyte maturation processes. Analysis of these genes allowed us to identify important new genes and noncoding RNAs potentially involved in COC maturation and cumulus expansion. These results may increase our understanding of the process of oocyte maturation and could ultimately improve the efficacy of IVM treatment.

Identifying regulatory mechanisms underlying tumorigenesis using locus expression signature analysis

Retroviral insertional mutagenesis is a powerful tool for identifying putative cancer genes in mice. To uncover the regulatory mechanisms by which common insertion loci affect downstream processes, we supplemented genotyping data with genome-wide mRNA expression profiling data for 97 tumors induced by retroviral insertional mutagenesis. We developed locus expression signature analysis, an algorithm to construct and interpret the differential gene expression signature associated with each common insertion locus. Comparing locus expression signatures to promoter affinity profiles allowed us to build a detailed map of transcription factors whose protein-level regulatory activity is modulated by a particular locus. We also predicted a large set of drugs that might mitigate the effect of the insertion on tumorigenesis. Taken together, our results demonstrate the potential of a locus-specific signature approach for identifying mammalian regulatory mechanisms in a cancer context.

ATM-dependent E2F1 accumulation in the nucleolus is an indicator of ribosomal stress in early response to DNA damage

The nucleolus plays a major role in ribosome biogenesis. Most genotoxic agents disrupt nucleolar structure and function, which results in the stabilization/activation of p53, inducing cell cycle arrest or apoptosis. Likewise, transcription factor E2F1 as a DNA damage responsive protein also plays roles in cell cycle arrest, DNA repair, or apoptosis in response to DNA damage through transcriptional response and protein-protein interaction. Furthermore, E2F1 is known to be involved in regulating rRNA transcription. However, how E2F1 displays in coordinating DNA damage and nucleolar stress is unclear. In this study, we demonstrate that ATM-dependent E2F1 accumulation in the nucleolus is a characteristic feature of nucleolar stress in early response to DNA damage. We found that at the early stage of DNA damage, E2F1 accumulation in the nucleolus was an ATM-dependent and a common event in p53-suficient and -deficient cells. Increased nucleolar E2F1 was sequestered by the nucleolar protein p14ARF, which repressed E2F1-dependent rRNA transcription initiation, and was coupled with S phase. Our data indicate that early accumulation of E2F1 in the nucleolus is an indicator for nucleolar stress and a component of ATM pathway, which presumably buffers elevation of E2F1 in the nucleoplasm and coordinates the diversifying mechanisms of E2F1 acts in cell cycle progression and apoptosis in early response to DNA damage.

A genotoxic stress-responsive miRNA, miR-574-3p, delays cell growth by suppressing the enhancer of rudimentary homolog gene in vitro

MicroRNA (miRNA) is a type of non-coding RNA that regulates the expression of its target genes by interacting with the complementary sequence of the target mRNA molecules. Recent evidence has shown that genotoxic stress induces miRNA expression, but the target genes involved and role in cellular responses remain unclear. We examined the role of miRNA in the cellular response to X-ray irradiation by studying the expression profiles of radio-responsive miRNAs and their target genes in cultured human cell lines. We found that expression of miR-574-3p was induced in the lung cancer cell line A549 by X-ray irradiation. Overexpression of miR-574-3p caused delayed growth in A549 cells. A predicted target site was detected in the 3′-untranslated region of the enhancer of the rudimentary homolog (ERH) gene, and transfected cells showed an interaction between the luciferase reporter containing the target sequences and miR-574-3p. Overexpression of miR-574-3p suppressed ERH protein production and delayed cell growth. This delay was confirmed by knockdown of ERH expression. Our study suggests that miR-574-3p may contribute to the regulation of the cell cycle in response to X-ray irradiation via suppression of ERH protein production.

ARF tumor suppression in the nucleolus

Since its discovery close to twenty years ago, the ARF tumor suppressor has played a pivotal role in the field of cancer biology. Elucidating ARF's basal physiological function in the cell has been the focal interest of numerous laboratories throughout the world for many years. Our current understanding of ARF is constantly evolving to include novel frameworks for conceptualizing the regulation of this critical tumor suppressor. As a result of this complexity, there is great need to broaden our understanding of the intricacies governing the biology of the ARF tumor suppressor. The ARF tumor suppressor is a key sensor of signals that instruct a cell to grow and proliferate and is appropriately localized in nucleoli to limit these processes. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

miR-34a is essential for p19(Arf)-driven cell cycle arrest

The Arf tumor suppressor gene product, p19(Arf), regulates cell proliferation in incipient cancer cells and during embryo development. Beyond its commonly accepted p53-dependent actions, p19(Arf) also acts independently of p53 in both contexts. One such p53-independent effect with in vivo relevance includes its repression of Pdgfrβ, a process that is essential for vision in the mouse. We have utilized cell culture-based and mouse models to define a new role for miR-34a in this process. Ectopic expression of Arf in cultured cells enhanced the expression of several microRNAs predicted to target Pdgfrß synthesis, including the miR-34 family. Because miR-34a has been implicated as a p53-dependent effector, we investigated whether it also contributed to p53-independent effects of p19(Arf). Indeed, in mouse embryo fibroblasts (MEFs) lacking p53, Arf-driven repression of Pdgfrβ and its blockade of Pdgf-B stimulated DNA synthesis were both completely interrupted by anti-microRNA against miR-34a. Ectopic miR-34a directly targeted Pdgfrβ and a plasmid reporter containing wild-type Pdgfrβ 3'UTR sequence, but not one in which the miR-34a target sequence was mutated. Although miR-34a expression has been linked to p53-a well-known effector of p19(Arf)-Arf expression and its knockdown correlated with miR-34a level in MEFs lacking p53. Finally, analysis of the mouse embryonic eye demonstrated that Arf controlled expression of miR-34a, and the related miR-34b and c, in vivo during normal mouse development. Our findings indicate that miR-34a provides an essential link between p19(Arf) and its p53-independent capacity to block cell proliferation driven by Pdgfrβ. This has ramifications for developmental and tumor suppressor roles of Arf.

Targeting the nucleolus for cancer intervention

The contribution of the nucleolus to cancer is well established with respect to its traditional role in facilitating ribosome biogenesis and proliferative capacity. More contemporary studies however, infer that nucleoli contribute a much broader role in malignant transformation. Specifically, extra-ribosomal functions of the nucleolus position it as a central integrator of cellular proliferation and stress signaling, and are emerging as important mechanisms for modulating how oncogenes and tumor suppressors operate in normal and malignant cells. The dependence of certain tumor cells to co-opt nucleolar processes to maintain their cancer phenotypes has now clearly been demonstrated by the application of small molecule inhibitors of RNA Polymerase I to block ribosomal DNA transcription and disrupt nucleolar function (Bywater et al., 2012 [1]). These drugs, which selectively kill tumor cells in vivo while sparing normal cells, have now progressed to clinical trials. It is likely that we have only just begun to scratch the surface of the potential of the nucleolus as a new target for cancer therapy, with "suppression of nucleolar stress" representing an emerging "hallmark" of cancer. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

p14ARF induces apoptosis via an entirely caspase-3-dependent mitochondrial amplification loop

The p14(ARF) tumor suppressor triggers cell death or cell cycle arrest upon oncogenic stress. In MCF-7 breast carcinoma cells, expression of the tumor suppressor gene p14(ARF) fails to trigger apoptosis but induces an arrest in the G1 and, to a lesser extent, in the G2 phase in the cell division cycle. Here, inhibition of cell cycle arrest resulted in apoptosis induction in caspase-3 proficient MCF-7 cells upon expression of p14(ARF) . This occurred in the absence of S-phase progression or mitotic entry. In contrast, syngeneic, caspase-3-deficient MCF-7 cells remained entirely resistant to p14(ARF) -induced apoptosis. Thus, cell cycle checkpoint abrogation overcomes resistance to p14(ARF) -induced cell death and promotes cell death via a caspase-3-dependent pathway. Cell death coincided with dissipation of the mitochondrial membrane potential, release of cytochrome c, and was inhibitable by pan-caspase inhibitors and the caspase-3/7 inhibitor zDEVD-fmk. Of note, mitochondrial events of apoptosis execution depended entirely on caspase-3 proficiency indicating that caspase-3 either acts "up-stream" of the mitochondria in a "non-canonical" pathway or mediates a mitochondrial feedback loop to amplify the apoptotic caspase signal in p14(ARF) -induced stress signaling.

Siva1 inhibits p53 function by acting as an ARF E3 ubiquitin ligase

The tumour suppressor alternative reading frame (ARF) is one of the most frequently mutated proteins in human cancer. It has been well established that ARF is able to stabilize and activate p53 by directly inhibiting Mdm2. ARF-mediated p53 activation in response to oncogenic stress is thought to be an important determinant of protection against cancer. However, little is known regarding the control of ARF in cells. Here, we show that Siva1 is a specific E3 ubiquitin ligase of ARF. Siva1 physically interacts with ARF both in vitro and in vivo. Through direct interaction, Siva1 promotes the ubiquitination and degradation of ARF, which in turn affects the stability of p53. Functionally, Siva1 regulates cell cycle progression and cell proliferation in an ARF/p53-dependent manner. Our results uncover a novel regulatory mechanism for the control of ARF stability, thereby revealing an important function of Siva1 in the regulation of the ARF-Mdm2-p53 pathway.

Rbf1 degron dysfunction enhances cellular DNA replication

The E2F family of transcription factors contributes to oncogenesis through activation of multiple genes involved in cellular proliferation, a process that is opposed by the Retinoblastoma tumor suppressor protein (RB). RB also increases E2F1 stability by inhibiting its proteasome-mediated degradation, but the consequences of this post-translational regulation of E2F1 remain unknown. To better understand the mechanism of E2F stabilization and its physiological relevance, we examined the streamlined Rbf1-dE2F1 network in Drosophila. During embryonic development, Rbf1 is insulated from ubiquitin-mediated turnover by the COP9 signalosome, a multi-protein complex that modulates E3 ubiquitin ligase activity. Here, we report that the COP9 signalosome also protects the Cullin4-E3 ligase that is responsible for dE2F1 proteasome-mediated destruction. This dual role of the COP9 signalosome may serve to buffer E2F levels, enhancing its turnover via Cul4 protection and its stabilization through protection of Rbf1. We further show that Rbf1-mediated stabilization of dE2F1 and repression of dE2F1 cell cycle-target genes are distinct properties. Removal of an evolutionarily conserved Rbf1 C terminal degron disabled Rbf1 repression without affecting dE2F1 stabilization. This mutant form of Rbf1 also enhanced G(1)-to-S phase progression when expressed in Rbf1-containing S2 embryonic cells, suggesting that such mutations may generate gain-of-function properties relevant to cellular transformation. Consistent with this idea, several studies have identified mutations in the homologous C terminal domains of RB and p130 in human cancer.

Hypermethylation-mediated silencing of p14(ARF) in fibroblasts from idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a devastating lung disease of unknown etiology. A conspicuous feature is the formation and persistence of fibroblastic/myofibroblastic foci throughout the lung parenchyma. Mechanisms remain unknown, but data indicate that fibroblasts acquire an antiapoptotic phenotype. We hypothesized that transcriptional silencing of proapoptotic genes may be implicated, and accordingly we evaluated the epigenetic regulation of p14(ARF). The expression of p14(ARF) was analyzed by RT-PCR in IPF (n = 8) and normal derived fibroblasts (n = 4) before and after treatment with 5-aza-2'-deoxycytidine (5-aza) and trichostatin A (TSA). p14(ARF) gene promoter methylation was determined by methylation-specific PCR (MS-PCR) and by DNA digestion with endonuclease McrBc, which cleaves 50% of methylated CpG. Apoptosis was evaluated by Annexin-V and nuclear staining. p14(ARF) expression was significantly decreased in four of the eight IPF fibroblasts lines, which was restored after 5-aza treatment. No changes were found with TSA. MS-PCR of bisulfite-treated genomic DNA showed a correlation between the reduced expression of p14(ARF) and the presence of hypermethylated promoter. No amplification was observed in the DNA treated with the McrBc enzyme, corroborating promoter hypermethylation. p14(ARF)-hypermethylated IPF fibroblasts were significantly more resistant to staurosporine-and S-nitrosoglutathione-induced apoptosis compared with normal and nonmethylated IPF fibroblasts (P < 0.01) and showed reduced levels of p53. Resistance to apoptosis was provoked in fibroblasts when p14(ARF) expression was inhibited by siRNA (P < 0.05). These findings demonstrate that many IPF fibroblasts have reduced expression of the proapoptotic p14(ARF) attributable to promoter hypermethylation and indicate that epigenetic mechanisms may underlie their resistance to apoptosis.

Identification of a checkpoint modulator with synthetic lethality to p53 mutants

The G(2) checkpoint is an indispensable pathway for cancers lacking p53 function, for delaying cell cycle progression, and for completing DNA repair. Therefore, disruption of this pathway is expected to offer selective therapy for these highly prevalent cancers. The aim of this study was to identify an inhibitor of the G(2) checkpoint including the ataxia-telangiectasia-mutated and Rad3-related checkpoint kinase 1 pathway that selectively suppresses the growth of p53-deficient cells. To obtain molecules with a novel mechanism of action, we constructed a high-throughput screening system that detected abrogation of the G(2) checkpoint in X-irradiated HT-29 cells. The screening resulted in identification of a guanidine analog, CBP-93872 that dose dependently inhibited the G(2) checkpoint induced by DNA damage. Interestingly, CBP-93872 directly suppressed the growth of p53-mutated cancer cell lines with wild-type CDKN2A by eliciting G(1) arrest, but not CDKN2A-deleted and/or wild-type p53 lines. CBP-93872 decreased phospho-cdc2 Y15 by inhibiting phosphorylation of Chk1, but did not suppress phospho-Chk2 or the kinase activities of either Chk1 or Chk2 in cellular or cell-free assays. These results suggest that a checkpoint modulator through suppression of Chk1 phosphorylation provides synthetic lethality to p53-deficient cells.

P14ARF inhibits human glioblastoma-induced angiogenesis by upregulating the expression of TIMP3

Malignant gliomas are the most common and the most lethal primary brain tumors in adults. Among malignant gliomas, 60%-80% show loss of P14ARF tumor suppressor activity due to somatic alterations of the INK4A/ARF genetic locus. The tumor suppressor activity of P14ARF is in part a result of its ability to prevent the degradation of P53 by binding to and sequestering HDM2. However, the subsequent finding of P14ARF loss in conjunction with TP53 gene loss in some tumors suggests the protein may have other P53-independent tumor suppressor functions. Here, we report what we believe to be a novel tumor suppressor function for P14ARF as an inhibitor of tumor-induced angiogenesis. We found that P14ARF mediates antiangiogenic effects by upregulating expression of tissue inhibitor of metalloproteinase-3 (TIMP3) in a P53-independent fashion. Mechanistically, this regulation occurred at the gene transcription level and was controlled by HDM2-SP1 interplay, where P14ARF relieved a dominant negative interaction of HDM2 with SP1. P14ARF-induced expression of TIMP3 inhibited endothelial cell migration and vessel formation in response to angiogenic stimuli produced by cancer cells. The discovery of this angiogenesis regulatory pathway may provide new insights into P53-independent P14ARF tumor-suppressive mechanisms that have implications for the development of novel therapies directed at tumors and other diseases characterized by vascular pathology.

p14(ARF)-induced apoptosis in p53 protein-deficient cells is mediated by BH3-only protein-independent derepression of Bak protein through down-regulation of Mcl-1 and Bcl-xL proteins

The p14(ARF) tumor suppressor plays a central role in regulating cell cycle arrest and apoptosis. We reported previously that p14(ARF) is capable of triggering apoptosis in a p53-independent manner. However, the mechanism remained unclear. Here we demonstrate that the p53-independent activation of the mitochondrial apoptosis pathway by p14(ARF) is primarily mediated by the pro-apoptotic Bax-homolog Bak. Expression of p14(ARF) exclusively triggers a N-terminal conformational switch of Bak, but not Bax, which allows for mitochondrial permeability shift, release of cytochrome c, activation of caspases, and subsequent fragmentation of genomic DNA. Although forced expression of Bak markedly sensitizes toward p14(ARF)-induced apoptosis, re-expression of Bax has no effect. Vice versa, knockdown of Bak by RNA interference attenuates p14(ARF)-induced apoptosis, whereas down-regulation of Bax has no effect. Bak activation coincides with a prominent, caspase-independent deprivation of the endogenous Bak inhibitors Mcl-1 and Bcl-x(L). In turn, mitochondrial apoptosis is fully blocked by overexpression of either Mcl-1 or Bcl-x(L). Taken together, these data indicate that in the absence of functional p53 and Bax, p14(ARF) triggers mitochondrial apoptosis signaling by activating Bak, which is facilitated by down-regulating anti-apoptotic Mcl-1 and Bcl-x(L). Moreover, our data suggest that the simultaneous inhibition of two central endogenous Bak inhibitors, i.e. Mcl-1 and Bcl-x(L), may be sufficient to activate mitochondrial apoptosis in the absence of BH3-only protein regulation.

RUVBL2 is a novel repressor of ARF transcription

ARF is the second most commonly inactivated tumor suppressor behind p53. It has been implicated in the control of cell proliferation, cell senescence, and tumor suppression. However, the detailed mechanism underlying the transcriptional control of ARF remains largely unknown. Here we report RUVBL2 as a novel transcriptional repressor of ARF. Ectopic expression of RUVBL2 decreases the levels of ARF, whereas knockdown of RUVBL2 results in a marked increase in ARF levels. In addition, RUVBL2 down-regulates the levels of p53 in an ARF-dependent manner. Mechanistically, RUVBL2 binds to the distal region of ARF promoter, thus leading to the repression of ARF transcription. These results suggest an important role of RUVBL2 in the regulation of ARF-p53 pathway.

Network calisthenics: control of E2F dynamics in cell cycle entry

Stimulation of quiescent mammalian cells with mitogens induces an abrupt increase in E2F1-3 expression just prior to the onset of DNA synthesis, followed by a rapid decline as replication ceases. This temporal adaptation in E2F facilitates a transient pattern of gene expression that reflects the ordered nature of DNA replication. The challenge to understand how E2F dynamics coordinate molecular events required for high-fidelity DNA replication has great biological implications. Indeed, precocious, prolonged, elevated or reduced accumulation of E2F can generate replication stress that culminates in either arrest or death. Accordingly, temporal characteristics of E2F are regulated by several network modules that include feedforward and autoregulatory loops. In this review, we discuss how these network modules contribute to "shaping" E2F dynamics in the context of mammalian cell cycle entry.

JAZ mediates G1 cell cycle arrest by interacting with and inhibiting E2F1

We discovered and reported JAZ as a unique dsRNA binding zinc finger protein that functions as a direct, positive regulator of p53 transcriptional activity to mediate G1 cell cycle arrest in a mechanism involving upregulation of the p53 target gene, p21. We now find that JAZ can also negatively regulate the cell cycle in a novel, p53-independent mechanism resulting from the direct interaction with E2F1, a key intermediate in regulating cell proliferation and tumor suppression. JAZ associates with E2F1's central DNA binding/dimerization region and its C-terminal transactivation domain. Functionally, JAZ represses E2F1 transcriptional activity in association with repression of cyclin A expression and inhibition of G1/S transition. This mechanism involves JAZ-mediated inhibition of E2F1's specific DNA binding activity. JAZ directly binds E2F1 in vitro in a dsRNA-independent manner, and JAZ's dsRNA binding ZF domains, which are necessary for localizing JAZ to the nucleus, are required for repression of transcriptional activity in vivo. Importantly for specificity, siRNA-mediated "knockdown" of endogenous JAZ increases E2F transcriptional activity and releases cells from G1 arrest, indicating a necessary role for JAZ in this transition. Although JAZ can directly inhibit E2F1 activity independently of p53, if functional p53 is expressed, JAZ may exert a more potent inhibition of cell cycle following growth factor withdrawal. Therefore, JAZ plays a dual role in cell cycle regulation by both repressing E2F1 transcriptional activity and activating p53 to facilitate efficient growth arrest in response to cellular stress, which may potentially be exploited therapeutically for tumor growth inhibition.

p14(ARF) inhibits the functions of adenovirus E1A oncoprotein

The tumour suppressor ARF (alternative reading frame) is one of the most important oncogenic stress sensors. ARF provides an 'oncogenic checkpoint' function through both p53-dependent and p53-independent mechanisms. In the present study, we demonstrate a novel p53-independent interaction between p14(ARF) and the adenovirus oncoprotein E1A. p14(ARF) inhibits E1A transcriptional function and promotes ubiquitination-dependent degradation of E1A. p14(ARF) overexpression relocalizes E1A into the nucleolus and inhibits E1A-induced cellular DNA replication independent of p53. Knockdown of endogenous p14(ARF) increases E1A transactivation. In addition, E1A can competitively inhibit ARF-Mdm2 (murine double minute 2) complex formation. These results identify a novel binding partner of p14(ARF) and reveal a mutually inhibitory interaction between p14(ARF) and E1A. We speculate that the ARF-E1A interaction may represent an additional host defence mechanism to limit viral replication. Alternatively, the interaction may allow adenovirus to sense the functional state of p53 in host cells, and fine-tune its own replication activity to prevent the triggering of a detrimental host response.

p14ARF interacts with E2F factors to form p14ARF-E2F/partner-DNA complexes repressing E2F-dependent transcription

Primarily, E2F factors such as E2F1, -2, and -3 stimulate cell-cycle progression, while ARF tumor suppressor mediates growth suppression. The ARF gene can be induced by oncogenic signal through activating E2F-dependent transcription. In turn, ARF may target E2F for its degradation via a p53-dependent mechanism. However, it remains unclear how the cell keeps the balance between the functional opposites of E2F and ARF. In this study, we demonstrate that p14ARF interacts with E2F1-3 factors to directly repress their transcriptional activities through forming p14ARF-E2F/partner-DNA super complexes, regardless of E2F protein degradation. The inhibition of E2F transcriptional activities by p14ARF in this manner occurs commonly in a variety of cell types, including p53-deficient and p53-wild type cells. Thus, E2F-mediated activation of the ARF gene and ARF-mediated functional inhibition of E2F compose a feedback loop, by which the two opposites act in concert to regulate cell proliferation and apoptosis, depending on the cellular context and the environment.

CDKN1C negatively regulates RNA polymerase II C-terminal domain phosphorylation in an E2F1-dependent manner

CDKN1C is a cyclin-dependent kinase inhibitor and is a candidate tumor suppressor gene. We previously found that the CDKN1C protein represses E2F1-driven transcription in an apparent negative feedback loop. Herein, we explore the mechanism by which CDKN1C represses transcription. We find that adenoviral-mediated overexpression of CDKN1C leads to a dramatic reduction in phosphorylation of the RNA polymerase II (pol II) C-terminal domain (CTD). RNA interference studies demonstrate that this activity is not an artifact of CDKN1C overexpression, because endogenous CDKN1C mediates an inhibition of RNA pol II CTD phosphorylation in HeLa cells upon treatment with dexamethasone. Surprisingly, we find that CDKN1C-mediated repression of RNA pol II phosphorylation is E2F1-dependent, suggesting that E2F1 may direct CDKN1C to chromatin. Chromatin immunoprecipitation assays demonstrate that CDKN1C is associated with E2F1-regulated promoters in vivo and that this association can dramatically reduce the level of RNA pol II CTD phosphorylation at both Ser-2 and Ser-5 of the C-terminal domain repeat. In addition, we show that CDKN1C interacts with both CDK7 and CDK9 (putative RNA pol II CTD kinases) and that CDKN1C blocks their ability to phosphorylate a glutathione S-transferase-CTD fusion protein in vitro. E2F1 and CDKN1C are found to form stable complexes both in vivo and in vitro. Molecular studies demonstrate that the E2F1-CDKN1C interaction is mediated by two E2F domains. A central E2F1 domain interacts directly with CDKN1C, whereas a C-terminal E2F1 domain interacts with CDKN1C via interaction with Rb. The results presented in this report highlight a novel mechanism of tumor suppression by CDKN1C.

p53 and E2f: partners in life and death

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

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

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

Deregulated expression of E2F1 promotes proteolytic degradation of tumor suppressor p73 and inhibits its transcriptional activity

The expression of tumor suppressor p73 is regulated at mRNA and protein levels. It has been shown that E2F1 acts as a transcriptional activator for p73. In this study, we have found that deregulated expression of E2F1 increases the mRNA level of p73, however, E2F1 promotes the degradation of p73. Immunoprecipitation experiments demonstrated that E2F1 forms a complex with p73 and inhibits the transcriptional activity of p73. Enforced expression of E2F1 induces degradation of p73 in a proteasome-independent manner. Additionally, the deletion analysis showed that E2F1(1-117) has an undetectable effect on p73, whereas E2F1(1-285) and E2F1(1-414) have an ability to promote degradation of p73 and inhibition of p73 transcriptional activity, suggesting that the region of E2F1 between amino acid residues 118 and 285 has a critical role in the regulation of p73. Taken together, our present study indicates that E2F1 has a dual role in the regulation of p73.

Cooperative effect of p21Cip1/WAF-1 and 14-3-3sigma on cell cycle arrest and apoptosis induction by p14ARF

P14(ARF) (p19(ARF) in the mouse) plays a central role in the regulation of cellular proliferation. Although the capacity of p14(ARF) to induce a cell cycle arrest in G1 phase depends on a functional p53/p21-signaling axis, the G2 arrest triggered by p14(ARF) is p53/p21-independent. Using isogeneic HCT116 cells either wild-type or homozygously deleted for p21, 14-3-3sigma or both, we further investigated the cooperative effect of p21 and 14-3-3sigma on cell cycle regulation and apoptosis induction by p14(ARF). In contrast to DNA damage, which induces mitotic catastrophe in 14-3-3sigma-deficient cells, we show here that the expression of p14(ARF) triggers apoptotic cell death, as evidenced by nuclear DNA fragmentation and induction of pan-caspase activities, irrespective of the presence or absence of 14-3-3sigma. The activation of the intrinsic mitochondrial apoptosis pathway by p14(ARF) was confirmed by cytochrome c release from mitochondria and induction of caspase-9- (LEHDase) and caspase-3/7-like (DEVDase) activities. Moreover, 14-3-3sigma/p21 double-deficient cells were exceedingly sensitive to apoptosis induction by p14(ARF) as compared to wild-type cells or cells lacking either gene alone. Notably, p14(ARF)-induced apoptosis was preceded by an arrest in the G2 phase of cell cycle, which coincided with downregulation of cdc2 (cdk1) protein expression and lack of its nuclear localization. This indicates that p14(ARF) impairs mitotic entry by targeting the distal DNA damage-signaling pathway and induces apoptotic cell death, rather than mitotic catastrophe, out of a transient G2 arrest. Furthermore, our data delineate that the disruption of G2/M cell cycle checkpoint control critically determines the sensitivity of the cell toward p14(ARF)-induced mitochondrial apoptosis.

The promiscuity of ARF interactions with the proteasome

The tumor suppressor ARF is one of the most important oncogenic stress sensors in mammalian cells. Its effect is exerted through the interaction with different cellular partners, often resulting in their functional inactivation. This review focuses on the role played by the proteasome in ARF regulation of protein turnover and the function of most of its interacting partners. Specific proteasome components appear to be involved in the regulation of ARF turnover, bringing to light a complex network of interactions between ARF and the proteasome.

Multimolecular complex of Par-4 and E2F1 binding to Smac promoter contributes to glutamate-induced apoptosis in human- bone mesenchymal stem cells

Neural cells undergo glutamate-induced apoptosis in ischaemic brain tissue, in which prostate apoptosis response-4 gene (Par-4) is involved. Human-bone mesenchymal stem cells can be utilized as an effective therapy for ischemic brain injury. In this study, we found that glutamate could induce apoptosis in human-bone mesenchymal stem cells, accompanied by increased expression of Par-4 gene and Smac release from mitochondria. Repressing Par-4 expression attenuated the glutamate-induced apoptosis. Both Par-4 protein and E2F1 protein could bind to E2F1-binding BS3 site on Smac promoter and participated in the formation of a proteins-DNA complex. Moreover, in the complex, E2F1, not Par-4, was found to be directly bound to the Smac promoter, suggesting that Par-4 exerted indirectly its transcriptional control on the Smac gene though interacting with E2F1. Expression of full-length Par-4 in human-bone mesenchymal cells resulted in increased activity of the Smac promoter. In addition, the indirect transcripional regulation of Par-4 on Smac depended on its COOH terminus-mediated interaction between Par-4 and E2F1. We conclude that the formation of proteins-DNA complex, containing Par-4 protein, E2F1 protein and the Smac promoter, contributes to the pro-apoptotic effect on glutamate-treated human-bone mesenchymal stem cells.

Melanomagenesis: overcoming the barrier of melanocyte senescence

Although melanoma ultimately progresses to a highly aggressive and metastatic disease that is typically resistant to currently available therapy, it often begins as a benign nevus consisting of a clonal population of hyperplastic melanocytes that cannot progress because they are locked in a state of cellular senescence. Once senescence is overcome, the nevus can exhibit dysplastic features and readily progress to more lethal stages. Recent advances have convincingly demonstrated that senescence represents a true barrier to the progression of many types of cancer, including melanoma. Thus, understanding the mechanism(s) by which melanoma evades senescence has become a priority in the melanoma research community. Senescence in most cells is regulated through some combination of activities within the RB and p53 pathways. However, differences discovered among various tumor types, some subtle and others quite profound, have revealed that senescence frequently operates in a context-dependent manner. Here we review what is known about melanocyte senescence, and how such knowledge may provide a much-needed edge in our struggles to contain or perhaps vanquish this often-fatal malignancy.

CARF: an emerging regulator of p53 tumor suppressor and senescence pathway

Replicative senescence, a major outcome of normal cells with finite lifespan, is a widely accepted in vitro model for ageing studies. Limited repair and defense mechanisms of normal cells, in addition to DNA alterations and oncogene inductions under stress, are believed to result in senescence as a protective mechanism to prevent undesirable proliferation of cells. The ARF/p53/p21(cip1/waf1) tumor suppression pathway acts as a molecular sensor and regulator of cellular stress, senescence, and immortalization. Understanding the molecular regulation of this pathway by intrinsic and extrinsic signals is extremely important to address unsolved questions in senescence and cancer. CARF was first discovered as a binding partner of ARF and has since been shown to have both ARF-dependent and -independent functions that converge to regulate p53 pathway. CARF directly binds to p53 and HDM2, and functions in a negative feedback pathway. Whereas CARF transcriptionally represses HDM2 to increase p53 activity, HDM2 in return degrades CARF. Thus, CARF may act as a novel key regulator of the p53 pathway at multiple checkpoints. The aim of this article is to discuss the current knowledge about functions of CARF and its impact on p53 pathway in regulation of senescence and carcinogenesis.

Characterization of a novel cytotoxic cell-penetrating peptide derived from p14ARF protein

The tumor suppressor p14ARF is widely deregulated in many types of cancers and is believed to function as a failsafe mechanism, inhibiting proliferation and inducing apoptosis as cellular response to a high oncogene load. We have found that a 22-amino-acid-long peptide derived from the N-terminal part of p14ARF, denoted ARF(1-22), which has previously been shown to mimic the function of p14ARF, has cell-penetrating properties. This peptide is internalized to the same extent as the cell-penetrating peptide (CPP) TP10 and dose-dependently decreases proliferation in MCF-7 and MDA MB 231 cells. Uptake of the ARF(1-22) peptide is associated with low membrane disturbance, measured by deoxyglucose and lactate dehydrogenase (LDH) leakage, as compared to its scrambled peptide. Also, flow cytometric analysis of annexin V/propidium iodide (PI) binding and Hoechst staining of nuclei suggest that ARF(1-22) induces apoptosis, whereas scrambled or inverted peptide sequences have no effect. The ARF(1-22) peptide mainly translocates cells through endocytosis, and is found intact inside cells for at least 3 hours. To our knowledge, this is the first time a CPP having pro-apoptopic activity has been designed from a protein.

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

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

E2F-1, Skp2 and cyclin E oncoproteins are upregulated and directly correlated in high-grade neuroendocrine lung tumors

The transcription factor E2F-1 plays a crucial role in the control of cellular growth. We previously reported its differential pattern of expression in human lung tumors. In this study, we have investigated the relationships linking the status of E2F-1 and a mediator of its proteasomal degradation, the S-phase kinase-associated protein 2 (Skp2) F-box protein. Using immunohistochemistry in a series of 129 lung tumors of all histological types, we demonstrate that Skp2 accumulates preferentially in high-grade neuroendocrine (HGNE) lung carcinomas (86%, P<0.0001), and show that Skp2 overexpression is associated with advanced stages (P<0.0001) and nodal metastasis (P<0.0001) in neuroendocrine (NE) lung tumors. Unexpectedly, we observe that Skp2 and E2F-1 expression directly correlates in NE lung tumors (P<0.0001). Moreover, using cellular models, we identify Skp2 as a new E2F-1 transcriptional target. Furthermore, we also provide evidence that Skp2 interacts physiologically with E2F-1 and stimulates its transcriptional activity toward the cyclin E promoter. Consistently, we demonstrate that cyclin E expression directly correlates with Skp2 (P<0.0001) and E2F-1 (P=0.0001) status in NE lung tumors. Overall, our data provide the first evidence of a direct and functional interconnection between the E2F-1, Skp2 and cyclin E oncoproteins in HGNE lung carcinomas.