Deregulated expression of E2F-1 induces S-phase entry and leads to apoptosis

The Interaction of Human Papillomavirus Infection and Prostaglandin E2 Signaling in Carcinogenesis: A Focus on Cervical Cancer Therapeutics

Chronic infection by high-risk human papillomaviruses (HPV) and chronic inflammation are factors associated with the onset and progression of several neoplasias, including cervical cancer. Oncogenic proteins E5, E6, and E7 from HPV are the main drivers of cervical carcinogenesis. In the present article, we review the general mechanisms of HPV-driven cervical carcinogenesis, as well as the involvement of cyclooxygenase-2 (COX-2)/prostaglandin E2 (PGE2) and downstream effectors in this pathology. We also review the evidence on the crosstalk between chronic HPV infection and PGE2 signaling, leading to immune response weakening and cervical cancer development. Finally, the last section updates the current therapeutic and preventive options targeting PGE2-derived inflammation and HPV infection in cervical cancer. These treatments include nonsteroidal anti-inflammatory drugs, prophylactic and therapeutical vaccines, immunomodulators, antivirals, and nanotechnology. Inflammatory signaling pathways are closely related to the carcinogenic nature of the virus, highlighting inflammation as a co-factor for HPV-dependent carcinogenesis. Therefore, blocking inflammatory signaling pathways, modulating immune response against HPV, and targeting the virus represent excellent options for anti-tumoral therapies in cervical cancer.

TFDP3 as E2F Unique Partner, Has Crucial Roles in Cancer Cells and Testis

Transcription factor DP family member 3 (TFDP3) is a cancer-testis antigen, mainly expressed in normal testis and multiple cancers. TFDP3 gene (Gene ID: 51270) is located on the chromosome X and shares a high degree of sequence homology with TFDP1 and TFDP2, which can form heterodimers with E2F family members and enhance DNA-binding activity of E2Fs. In contrast to TFDP1 and TFDP2, TFDP3 downregulates E2F-mediated transcriptional activation. During DNA damage response in cancer cells, TFDP3 is induced and can inhibit E2F1-mediated apoptosis. Moreover, TFDP3 is involved in cell autophagy and epithelial-mesenchymal transition. Regarding cancer therapy opportunity, the transduction of dendritic cells with recombinant adenovirus-encoding TFDP3 can activate autologous cytotoxic T lymphocytes to target hepatoma cells. Here, we review the characterization of TFDP3, with an emphasis on the biological function and molecular mechanism. A better understanding of TFDP3 will provide new insights into the pathological mechanisms and therapeutic strategies for cancers.

PRR11 promotes ccRCC tumorigenesis by regulating E2F1 stability

Proline rich 11 (PRR11), a novel tumor-related gene, has been identified in different tumors. However, the relevant biological functions of PRR11 in human clear cell renal cell carcinoma (ccRCC) have not been studied. In this study, we first identified PRR11 as a biomarker of ccRCC and predictor of poor prognosis by bioinformatics. Then, we showed that PRR11 silencing substantially reduced ccRCC cell proliferation and migration in vitro and in vivo. Importantly, we found that PRR11 induced the degradation of the E2F1 protein through its interaction with E2F1, and PRR11 reduced the stability of the E2F1 protein in ccRCC cells, thereby affecting cell cycle progression. Further results indicated that the downregulation of E2F1 expression partially reversed the changes in ccRCC cell biology caused by PRR11 deletion. In addition, we showed that PRR11 was a target gene of c-Myc. The transcription factor c-Myc may have promoted the expression of PRR11 in ccRCC cells by binding to the PRR11 promoter region, thereby accelerating the progression of ccRCC. In summary, we found that PRR11 served as an oncogene in ccRCC, and PRR11 reduced the protein stability of E2F1 and could be activated by c-Myc.

Evolutionarily Conserved Long Non-coding RNA Regulates Gene Expression in Cytokine Storm During COVID-19

Coronavirus is a family of viruses including alpha-, beta-, gamma-, delta-coronaviruses. Only alpha- and betacoronaviruses have been observed to infect humans. Past outbreaks of SARS-CoV and MERS-CoV, both betacoronavirus, are the result of a spillover from animals. Recently, a new strain termed SARS-CoV-2 emerged in December 2019 in Wuhan, China. Severe cases of COVID-19, the disease caused by SARS-CoV-2, lead to acute respiratory distress syndrome (ARDS). One contributor to the development of ARDS is cytokine storm, an overwhelming inflammatory immune response. Long non-coding RNAs (lncRNAs) are genetic regulatory elements that, among many functions, alter gene expression and cellular processes. lncRNAs identified to be pertinent in COVID-19 cytokine storm have the potential to serve as disease markers or drug targets. This project aims to computationally identify conserved lncRNAs potentially regulating gene expression in cytokine storm during COVID-19. We found 22 lncRNAs that can target 10 cytokines overexpressed in COVID-19 cytokine storm, 8 of which targeted two or more cytokine storm cytokines. In particular, the lncRNA non-coding RNA activated by DNA damage (NORAD), targeted five out of the ten identified cytokine storm cytokines, and is evolutionarily conserved across multiple species. These lncRNAs are ideal candidates for further in vitro and in vivo analysis.

Regulation of E2F1 activity via PKA-mediated phosphorylations

E2F1 becomes activated during the G1 phase of the cell cycle, and posttranslational modifications modulate its activity. Activation of G-protein coupled receptors (GPCR) by many ligands induces the activation of adenylate cyclases and the production of cAMP, which activates the PKA enzyme. Activated PKA elicits its biological effect by phosphorylating the target proteins containing serine or threonine amino acids in the RxxS/T motif. Since PKA activation negatively regulates cell proliferation, we thought that activated PKA would negatively affect the activity of E2F1. In line with this, when we analyzed the amino acid sequence of E2F1, we found 3 hypothetical consensus PKA phosphorylation sites located at 127-130, 232-235, and 361-364 positions and RYET, RLLS, and RMGS sequences. After showing the binding and phosphorylation of E2F1 by PKA, we converted the codons of Threonine-130, Serine-235, and Serine-364 to Alanine and Glutamic acid codons on the eukaryotic E2F1 expression vector we had previously created. We confirmed the phosphorylation of T130, S235, and S364 by developing monoclonal antibodies against phospho-specific forms of these sites and showed that their phosphorylation is cell cycle-dependent. According to our results, PKA-mediated phosphorylation of E2F1 by PKA inhibits proliferation and glucose uptake and induces caspase-3 activation and senescence.

An integrated multi-omics approach identifies epigenetic alterations associated with Alzheimer's disease

Protein aggregation is the hallmark of neurodegeneration but the molecular mechanisms underlying late-onset Alzheimer’s disease (AD) remain unclear. Here we integrated transcriptomic, proteomic and epigenomic analyses of post-mortem human brains to identify molecular pathways involved in AD. RNA-seq analysis revealed upregulation of transcription- and chromatin-related genes, including the histone acetyltransferases for H3K27ac and H3K9ac. An unbiased proteomic screening singled out H3K27ac and H3K9ac as main enrichments specific to AD. In turn, epigenomic profiling revealed gains of H3K27ac and H3K9ac linked to transcription, chromatin, and disease pathways in AD. Increasing genome-wide H3K27ac and H3K9ac in a fly model of AD exacerbated amyloid-β42-driven neurodegeneration. Together, these findings suggest that AD involves a reconfiguration of the epigenome, where H3K27ac and H3K9ac impact disease pathways by dysregulating transcription- and chromatin-gene feedback loops. The identification of this process highlights potential epigenetic strategies for early-stage disease treatment.

PTEN Inhibition Protects Against Experimental Intracerebral Hemorrhage-Induced Brain Injury Through PTEN/E2F1/β-Catenin Pathway

Intracerebral hemorrhage (ICH) is a subtype of stroke with highest mortality and morbidity. We have previously demonstrated that dipotassium bisperoxo (picolinato) oxovanadate (V), (bpV[pic]) inhibits phosphatase and tensin homolog (PTEN) and activates extracellular signal-regulated kinase (ERK)1/2. In this study, we examined the effect of bpV[pic] in the rat ICH model in vivo and the hemin-induced injury model in rat cortical cultures. The rat model of ICH was created by injecting autologous blood into the striatum, and bpV[pic] was intraperitoneally injected. The effects of bpV[pic] were evaluated by neurological tests, Fluoro-Jade C (FJC) staining, and Nissl staining. We demonstrate that bpV[pic] attenuates ICH-induced brain injury in vivo and hemin-induced neuron injury in vitro. The expression of E2F1 was increased, but β-catenin expression was decreased after ICH, and the altered expressions of E2F1 and β-catenin after ICH were blocked by bpV[pic] treatment. Our results further show that bpV[pic] increases β-catenin expression through downregulating E2F1 in cortical neurons and prevents hemin-induced neuronal damage through E2F1 downregulation and subsequent upregulation of β-catenin. By testing the effect of PTEN-siRNA, PTEN cDNA, or combined use of ERK1/2 inhibitor and bpV[pic] in cultured cortical neurons after hemin-induced injury, we provide evidence suggesting that PTEN inhibition by bpV[pic] confers neuroprotection through E2F1 and β-catenin pathway, but the neuroprotective role of ERK1/2 activation by bpV[pic] cannot be excluded.

Role of E2Fs and mitotic regulators controlled by E2Fs in the epithelial to mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is a complex cellular process in which epithelial cells acquire mesenchymal properties. EMT occurs in three biological settings: development, wound healing and fibrosis, and tumor progression. Despite occurring in three independent biological settings, EMT signaling shares some molecular mechanisms that allow epithelial cells to de-differentiate and acquire mesenchymal characteristics that confer cells invasive and migratory capacity to distant sites. Here we summarize the molecular mechanism that delineates EMT and we will focus on the role of E2 promoter binding factors (E2Fs) in EMT during tumor progression. Since the E2Fs are presently undruggable due to their control in numerous pivotal cellular functions and due to the lack of selectivity against individual E2Fs, we will also discuss the role of three mitotic regulators and/or mitotic kinases controlled by the E2Fs (NEK2, Mps1/TTK, and SGO1) in EMT that can be useful as drug targets.

Impact statement

The study of the epithelial to mesenchymal transition (EMT) is an active area of research since it is one of the early intermediates to invasion and metastasis—a state of the cancer cells that ultimately kills many cancer patients. We will present in this review that besides their canonical roles as regulators of proliferation, unregulated expression of the E2F transcription factors may contribute to cancer initiation and progression to metastasis by signaling centrosome amplification, chromosome instability, and EMT. Since our discovery that the E2F activators control centrosome amplification and mitosis in cancer cells, we have identified centrosome and mitotic regulators that may represent actionable targets against EMT and metastasis in cancer cells. This is impactful to all of the cancer patients in which the Cdk/Rb/E2F pathway is deregulated, which has been estimated to be most cancer patients with solid tumors.

Differential requirement for dimerization partner DP between E2F-dependent activation of tumor suppressor and growth-related genes

The transcription factor E2F plays crucial roles in cell proliferation and tumor suppression by activating growth-related genes and pro-apoptotic tumor suppressor genes, respectively. It is generally accepted that E2F binds to target sequences with its heterodimeric partner DP. Here we show that, while knockdown of DP1 expression inhibited ectopic E2F1- or adenovirus E1a-induced expression of the CDC6 gene and cell proliferation, knockdown of DP1 and DP2 expression did not affect ectopic E2F1- or E1a-induced expression of the tumor suppressor ARF gene, an upstream activator of the tumor suppressor p53, activation of p53 or apoptosis. These observations suggest that growth related and pro-apoptotic E2F targets are regulated by distinct molecular mechanisms and contradict the threshold model, which postulates that E2F activation of pro-apoptotic genes requires a higher total activity of activator E2Fs, above that necessary for E2F-dependent activation of growth-related genes.

E2F1-regulated long non-coding RNA RAD51-AS1 promotes cell cycle progression, inhibits apoptosis and predicts poor prognosis in epithelial ovarian cancer

Long non-coding RNA RAD51 antisense RNA 1 (RAD51-AS1, also known as TODRA) has been shown to be down-regulated by E2F1, a key cell cycle and apoptosis regulator, in breast cancer. Little is known regarding the role of RAD51-AS1 in disease. Here, we investigate the role of RAD51-AS1 in epithelial ovarian cancer (EOC). Using luciferase reporter and chromatin immunoprecipitation experiments, we verified RAD51-AS1 as a target of E2F1 under negative regulation in EOC. We then examined RAD51-AS1 expression in EOC samples using in situ hybridization (ISH). RAD51-AS1 was localized to the nucleus and found to be a critical marker for clinical features that significantly correlated with poor survival in EOC patients. RAD51-AS1 was also an independent prognostic factor for EOC. Overexpression of RAD51-AS1 promoted EOC cell proliferation, while silencing of RAD51-AS1 inhibited EOC cell proliferation, delayed cell cycle progression and promoted apoptosis in vitro and in vivo. RAD51-AS1 may participate in carcinogenesis via regulation of p53 and p53-related genes. Our study highlights the role of RAD51-AS1 as a prognostic marker of EOC. Based on its regulation of the tumor suppressor p53, RAD51-AS1-based therapy may represent a viable therapeutic option for EOC in the near future.

Cytokinesis Failure Leading to Chromosome Instability in v-Src-Induced Oncogenesis

v-Src, an oncogene found in Rous sarcoma virus, is a constitutively active variant of c-Src. Activation of Src is observed frequently in colorectal and breast cancers, and is critical in tumor progression through multiple processes. However, in some experimental conditions, v-Src causes growth suppression and apoptosis. In this review, we highlight recent progress in our understanding of cytokinesis failure and the attenuation of the tetraploidy checkpoint in v-Src-expressing cells. v-Src induces cell cycle changes—such as the accumulation of the 4N cell population—and increases the number of binucleated cells, which is accompanied by an excess number of centrosomes. Time-lapse analysis of v-Src-expressing cells showed that cytokinesis failure is caused by cleavage furrow regression. Microscopic analysis revealed that v-Src induces delocalization of cytokinesis regulators including Aurora B and Mklp1. Tetraploid cell formation is one of the causes of chromosome instability; however, tetraploid cells can be eliminated at the tetraploidy checkpoint. Interestingly, v-Src weakens the tetraploidy checkpoint by inhibiting the nuclear exclusion of the transcription coactivator YAP, which is downstream of the Hippo pathway and its nuclear exclusion is critical in the tetraploidy checkpoint. We also discuss the relationship between v-Src-induced chromosome instability and growth suppression in v-Src-induced oncogenesis.

Expression level is a key determinant of E2F1-mediated cell fate

The Rb/E2F network has a critical role in regulating cell cycle progression and cell fate decisions. It is dysfunctional in virtually all human cancers, because of genetic lesions that cause overexpression of activators, inactivation of repressors, or both. Paradoxically, the downstream target of this network, E2F1, is rarely strongly overexpressed in cancer. E2F1 can induce both proliferation and apoptosis but the factors governing these critical cell fate decisions remain unclear. Previous studies have focused on qualitative mechanisms such as differential cofactors, posttranslational modification or state of other signaling pathways as modifiers of the cell fate decisions downstream of E2F1 activation. In contrast, the importance of the expression levels of E2F1 itself in dictating the downstream phenotypes has not been rigorously studied, partly due to the limited resolution of traditional population-level measurements. Here, through single-cell quantitative analysis, we demonstrate that E2F1 expression levels have a critical role in determining the fate of individual cells. Low levels of exogenous E2F1 promote proliferation, moderate levels induce G1, G2 and mitotic cell cycle arrest, and very high levels promote apoptosis. These multiple anti-proliferative mechanisms result in a strong selection pressure leading to rapid elimination of E2F1-overexpressing cells from the population. RNA-sequencing and RT-PCR revealed that low levels of E2F1 are sufficient to induce numerous cell cycle-promoting genes, intermediate levels induce growth arrest genes (i.e., p18, p19 and p27), whereas higher levels are necessary to induce key apoptotic E2F1 targets APAF1, PUMA, HRK and BIM. Finally, treatment of a lung cancer cell line with a proteasome inhibitor, MLN2238, resulted in an E2F1-dependent mitotic arrest and apoptosis, confirming the role of endogenous E2F1 levels in these phenotypes. The strong anti-proliferative activity of moderately overexpressed E2F1 in multiple cancer types suggests that targeting E2F1 for upregulation may represent an attractive therapeutic strategy in cancer.

Cell cycle inhibition reduces inflammatory responses, neuronal loss, and cognitive deficits induced by hypobaria exposure following traumatic brain injury

Background

Traumatic brain injury (TBI) patients in military settings can be exposed to prolonged periods of hypobaria (HB) during aeromedical evacuation. Hypobaric exposure, even with supplemental oxygen to prevent hypoxia, worsens outcome after experimental TBI, in part by increasing neuroinflammation. Cell cycle activation (CCA) after TBI has been implicated as a mechanism contributing to both post-traumatic cell death and neuroinflammation. Here, we examined whether hypobaric exposure in rats subjected to TBI increases CCA and microglial activation in the brain, as compared to TBI alone, and to evaluate the ability of a cyclin-dependent kinase (CDK) inhibitor (CR8) to reduce such changes and improve behavioral outcomes.

Methods

Adult male Sprague Dawley rats were subjected to fluid percussion-induced injury, and HB exposure was performed at 6 h after TBI. Western blot and immunohistochemistry (IHC) were used to assess cell cycle-related protein expression and inflammation at 1 and 30 days after injury. CR8 was administered intraperitoneally at 3 h post-injury; chronic functional recovery and histological changes were assessed.

Results

Post-traumatic hypobaric exposure increased upregulation of cell cycle-related proteins (cyclin D1, proliferating cell nuclear antigen, and CDK4) and microglial/macrophage activation in the ipsilateral cortex at day 1 post-injury as compared to TBI alone. Increased immunoreactivity of cell cycle proteins, as well as numbers of Iba-1⁺ and GFAP⁺ cells in both the ipsilateral cortex and hippocampus were found at day 30 post-injury. TBI/HB significantly increased the numbers of NADPH oxidase 2 (gp91^phox) enzyme-expressing cells that were co-localized with Iba-1⁺. Each of these changes was significantly reduced by the administration of CR8. Unbiased stereological assessment showed significantly decreased numbers of microglia displaying the highly activated phenotype in the ipsilateral cortex of TBI/HB/CR8 rats compared with TBI/HB/Veh rats. Moreover, treatment with this CDK inhibitor also significantly improved spatial and retention memory and reduced lesion volume and hippocampal neuronal cell loss.

Conclusions

HB exposure following TBI increases CCA, neuroinflammation, and associated neuronal cell loss. These changes and post-traumatic cognitive deficits are reduced by CDK inhibition; such drugs may therefore serve to protect TBI patients requiring aeromedical evacuation.

Redox regulation of FoxO transcription factors

Transcription factors of the forkhead box, class O (FoxO) family are important regulators of the cellular stress response and promote the cellular antioxidant defense. On one hand, FoxOs stimulate the transcription of genes coding for antioxidant proteins located in different subcellular compartments, such as in mitochondria (i.e. superoxide dismutase-2, peroxiredoxins 3 and 5) and peroxisomes (catalase), as well as for antioxidant proteins found extracellularly in plasma (e.g., selenoprotein P and ceruloplasmin). On the other hand, reactive oxygen species (ROS) as well as other stressful stimuli that elicit the formation of ROS, may modulate FoxO activity at multiple levels, including posttranslational modifications of FoxOs (such as phosphorylation and acetylation), interaction with coregulators, alterations in FoxO subcellular localization, protein synthesis and stability. Moreover, transcriptional and posttranscriptional control of the expression of genes coding for FoxOs is sensitive to ROS. Here, we review these aspects of FoxO biology focusing on redox regulation of FoxO signaling, and with emphasis on the interplay between ROS and FoxOs under various physiological and pathophysiological conditions. Of particular interest are the dual role played by FoxOs in cancer development and their key role in whole body nutrient homeostasis, modulating metabolic adaptations and/or disturbances in response to low vs. high nutrient intake. Examples discussed here include calorie restriction and starvation as well as adipogenesis, obesity and type 2 diabetes.

Api5 contributes to E2F1 control of the G1/S cell cycle phase transition

Background

The E2f transcription factor family has a pivotal role in controlling the cell fate in general, and in particular cancer development, by regulating the expression of several genes required for S phase entry and progression through the cell cycle. It has become clear that the transcriptional activation of at least one member of the family, E2F1, can also induce apoptosis. An appropriate balance of positive and negative regulators appears to be necessary to modulate E2F1 transcriptional activity, and thus cell fate.

Methodology/Principal Findings

In this report, we show that Api5, already known as a regulator of E2F1 induced-apoptosis, is required for the E2F1 transcriptional activation of G1/S transition genes, and consequently, for cell cycle progression and cell proliferation. Api5 appears to be a cell cycle regulated protein. Removal of Api5 reduces cyclin E, cyclin A, cyclin D1 and Cdk2 levels, causing G1 cell cycle arrest and cell cycle delay. Luciferase assays established that Api5 directly regulates the expression of several G1/S genes under E2F1 control. Using protein/protein and protein/DNA immunoprecipitation studies, we demonstrate that Api5, even if not physically interacting with E2F1, contributes positively to E2F1 transcriptional activity by increasing E2F1 binding to its target promoters, through an indirect mechanism.

Conclusion/Significance

The results described here support the pivotal role of cell cycle related proteins, that like E2F1, may act as tumor suppressors or as proto-oncogenes during cancer development, depending on the behavior of their positive and negative regulators. According to our findings, Api5 contributes to E2F1 transcriptional activation of cell cycle-associated genes by facilitating E2F1 recruitment onto its target promoters and thus E2F1 target gene transcription.

Flipping the switch from g1 to s phase with e3 ubiquitin ligases

The cell cycle ensures genome maintenance by coordinating the processes of DNA replication and chromosome segregation. Of particular importance is the irreversible transition from the G1 phase of the cell cycle to S phase. This transition marks the switch from preparing chromosomes for replication ("origin licensing") to active DNA synthesis ("origin firing"). Ubiquitin-mediated proteolysis is essential for restricting DNA replication to only once per cell cycle and is the major mechanism regulating the G1 to S phase transition. Although some changes in protein levels are attributable to regulated mRNA abundance, protein degradation elicits very rapid changes in protein abundance and is critical for the sharp and irreversible transition from one cell cycle stage to the next. Not surprisingly, regulation of the G1-to-S phase transition is perturbed in most cancer cells, and deregulation of key molecular events in G1 and S phase drives not only cell proliferation but also genome instability. In this review we focus on the mechanisms by which E3 ubiquitin ligases control the irreversible transition from G1 to S phase in mammalian cells.

Striking a balance: autophagy, apoptosis, and necrosis in a normal and failing heart

Despite the progress that has been made over the past two decades in cardiovascular research, heart failure remains a major cause of morbidity and mortality worldwide. Insight into the cellular and molecular mechanisms that underlie the heart failure in individuals with ischemic heart disease have identified defects in cellular processes that govern autophagy, apoptosis and necrosis as a prevailing underlying cause. Indeed, programmed cell death of cardiac cells by apoptosis or necrosis is believed to involve the intrinsic mitochondrial pathway and/or extrinsic death receptor pathway by certain Bcl-2 family members as well as components of the TNFα signaling pathway. In this review, we discuss recent advances in the molecular signaling factors that govern cardiac cell fate under normal and disease conditions.

MicroRNA-93 controls perfusion recovery after hindlimb ischemia by modulating expression of multiple genes in the cell cycle pathway

BACKGROUND

MicroRNAs are key regulators of gene expression in response to injury, but there is limited knowledge of their role in ischemia-induced angiogenesis, such as in peripheral arterial disease. Here, we used an unbiased strategy and took advantage of different phenotypic outcomes that follow surgically induced hindlimb ischemia between inbred mouse strains to identify key microRNAs involved in perfusion recovery from hindlimb ischemia.

METHODS AND RESULTS

From comparative microRNA profiling between inbred mouse strains that display profound differences in their extent of perfusion recovery after hindlimb ischemia, we found that the mouse strain with higher levels of microRNA-93 (miR-93) in hindlimb muscle before ischemia and the greater ability to upregulate miR-93 in response to ischemia had better perfusion recovery. In vitro, overexpression of miR-93 attenuated hypoxia-induced apoptosis in both endothelial and skeletal muscle cells and enhanced proliferation in both cell types. In addition, miR-93 overexpression enhanced endothelial cell tube formation. In vivo, miR-93 overexpression enhanced capillary density and perfusion recovery from hindlimb ischemia, and antagomirs to miR-93 attenuated perfusion recovery. Both in vitro and in vivo modulation of miR-93 resulted in alterations in the expression of >1 cell cycle pathway gene in 2 different cell types.

CONCLUSIONS

Our data indicate that miR-93 enhances perfusion recovery from hindlimb ischemia by modulation of multiple genes that coordinate the functional pathways of cell proliferation and apoptosis. Thus, miR-93 is a strong potential target for pharmacological modulation to promote angiogenesis in ischemic tissue.

E2f1-deficient NOD/SCID mice have dry mouth due to a change of acinar/duct structure and the down-regulation of AQP5 in the salivary gland

Non-obese diabetic (NOD) mice have been used as a model for dry mouth. NOD mice lacking the gene encoding E2f1, a transcription factor, develop hyposalivation more rapidly progressively than control NOD mice. However, the model mice are associated with an underlying disease such as diabetes. We have now established E2f1-deficient NOD/severe combined immunodeficiency disease (NOD/SCID.E2f1(-/-)) mice to avoid the development of diabetes (Matsui-Inohara et al., Exp Biol Med (Maywood) 234(12):1525-1536, 2009). In this study, we investigated the pathophysiological features of dry mouth using NOD/SCID.E2f1(-/-) mice. In NOD/SCID.E2f1(-/-) mice, the volume of secreted saliva stimulated with pilocarpine is about one third that of control NOD/SCID mice. In behavioral analysis, NOD/SCID.E2f1(-/-) mice drank plenty of water when they ate dry food, and the frequency and time of water intake were almost double compared with control NOD/SCID mice. Histological analysis of submandibular glands with hematoxylin-eosin stain revealed that NOD/SCID.E2f1(-/-) mice have more ducts than NOD/SCID mice. In western blot analysis, the expression of aquaporin 5 (AQP5), a marker of acinar cells, in parotid and in submandibular glands of NOD/SCID.E2f1(-/-) mice was lower than in NOD/SCID mice. Immunohistochemical analysis of parotid and submandibular acini revealed that the localization of AQP5 in NOD/SCID.E2f1(-/-) mice differs from that in NOD/SCID mice; AQP5 was leaky and diffusively localized from the apical membrane to the cytosol in NOD/SCID.E2f1(-/-) mice. The ubiquitination of AQP5 was detected in submandibular glands of NOD/SCID.E2f1(-/-) mice. These findings suggest that the change of acinar/duct structure and the down-regulation of AQP5 in the salivary gland cause the pathogenesis of hyposalivation in NOD/SCID.E2f1(-/-) mice.

E2F-1 lacking the transcriptional activity domain induces autophagy

The transcription factor E2F-1 plays a crucial role in the control of cell proliferation. E2F-1 has tumor suppressive properties by inducing apoptosis and autophagy. In this study, E2F-1 and its truncated form (E2Ftr), lacking the transactivation domain (TAD), were compared for their ability to induce autophagy. In Gaussia luciferase-based assays, both E2F-1 and E2Ftr induced the proteolytic cleavage of the autophagic marker LC3. In addition, LC3 and autophagy protein 5 (Atg5) were upregulated by E2F-1 and E2Ftr. Likewise, both E2F proteins induced a punctate pattern of GFP-tagged LC3, indicating autophagosome formation. The presence of double-membrane autophagic vesicles induced by E2F-1 and E2Ftr was confirmed by transmission electron microscopy (TEM). The application of z-VAD-fmk, a caspase inhibitor, partially blocked both E2F-1 and E2Ftr-mediated cytotoxicity. Moreover, Atg5 (-/-) cells were more resistant to the E2F-1 or E2Ftr-induced cell killing effect than Atg5 wt cells. The TAD of E2F-1 is not essential for induction of autophagy; apoptosis and autophagy cooperate for an efficient cancer cell killing effect induced by E2F-1 or E2Ftr. E2Ftr-induced autophagy is a promising approach to destroy tumors that are resistant to conventional treatments.

Src tyrosine kinase inhibits apoptosis through the Erk1/2- dependent degradation of the death accelerator Bik

Src, the canonical member of the non-receptor family of tyrosine kinases, is deregulated in numerous cancers, including colon and breast cancers. In addition to its effects on cell proliferation and motility, Src is often considered as an inhibitor of apoptosis, although this remains controversial. Thus, whether the ability of Src to generate malignancies relies on an intrinsic aptitude to inhibit apoptosis or requires preexistent resistance to apoptosis remains somewhat elusive. Here, using mouse fibroblasts transformed with v-Src as a model, we show that the observed Src-dependent resistance to cell death relies on Src ability to inhibit the mitochondrial pathway of apoptosis by specifically increasing the degradation rate of the BH3-only protein Bik. This effect relies on the activation of the Ras-Raf-Mek1/2-Erk1/2 pathway, and on the phosphorylation of Bik on Thr124, driving Bik ubiquitylation on Lys33 and subsequent degradation by the proteasome. Importantly, in a set of human cancer cells with Src-, Kras- or BRAF-dependent activation of Erk1/2, resistances to staurosporine or thapsigargin were also shown to depend on Bik degradation rate via a similar mechanism. These results suggest that Bik could be a rate-limiting factor for apoptosis induction of tumor cells exhibiting deregulated Erk1/2 signaling, which may provide new opportunities for cancer therapies.

Roles of salivary components in Streptococcus mutans colonization in a new animal model using NOD/SCID.e2f1-/- mice

Streptococcus mutans plays an important role in biofilm formation on the tooth surface and is the primary causative agent of dental caries. The binding of S. mutans to the salivary pellicle is of considerable etiologic significance and is important in biofilm development. Recently, we produced NOD/SCID.e2f1(-/-) mice that show hyposalivation, lower salivary antibody, and an extended life span compared to the parent strain: NOD.e2f1(-/-). In this study we used NOD/SCID.e2f1(-/-) 4 or 6 mice to determine the roles of several salivary components in S. mutans colonization in vivo. S. mutans colonization in NOD/SCID.e2f1(-/-) mice was significantly increased when mice were pre-treated with human saliva or commercial salivary components. Interestingly, pre-treatment with secretory IgA (sIgA) at physiological concentrations promoted significant colonization of S. mutans compared with sIgA at higher concentrations, or with human saliva or other components. Our data suggest the principal effects of specific sIgA on S. mutans occur during S. mutans colonization, where the appropriate concentration of specific sIgA may serve as an anti-microbial agent, agglutinin, or an adherence receptor to surface antigens. Further, specific sIgA supported biofilm formation when the mice were supplied 1% sucrose water and a non-sucrose diet. The data suggests that there are multiple effects exerted by sIgA in S. mutans colonization, with synergistic effects evident under the condition of sIgA and limited nutrients on colonization in NOD/SCID.e2f1(-/-) mice. This is a new animal model that can be used to assess prevention methods for dental biofilm-dependent diseases such as dental caries.

The role of E2F-1 and downstream target genes in mediating ischemia/reperfusion injury in vivo

E2Fs are a family of transcription factors that regulate proliferation, differentiation and apoptosis in many cell types. E2F-1 is the prototypical E2F and the family member that has most often been implicated in also mediating apoptosis. To better understand the role of E2F-1 in mediating cardiomyocyte injury we initially analyzed E2F family member expression after ischemia/reperfusion (I/R) in vivo or simulated ischemia in vitro. I/R injury in vivo caused a 3.4-fold increase specifically in E2F-1 protein levels. Expression of other E2F family members did not change. To establish the role of E2F-1 in I/R we examined the response of germline deleted E2F-1 mice to I/R injury. Infarct size as a percentage of the area at risk was decreased 39.8% in E2F-1(-/-) mice compared to E2F-1(+/+) controls. Interestingly, expression of classic, E2F-1 apoptotic target genes was not altered in E2F-1 null cardiomyocytes after I/R. However, upregulation of the primary member of the Forkhead family of transcription factors, FoxO-1a, was attenuated. Consistent, with a role for FoxO-1a as an important target of E2F-1 in I/R, a number of proapoptotic FoxO-1a target genes were also altered. These results suggest that E2F-1 and FoxO-1a belong to a complex transcriptional network that may modulate myocardial cell death during I/R injury.

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.

Adenovirus E1A directly targets the E2F/DP-1 complex

Deregulation of the cell cycle is of paramount importance during adenovirus infection. Adenovirus normally infects quiescent cells and must initiate the cell cycle in order to propagate itself. The pRb family of proteins controls entry into the cell cycle by interacting with and repressing transcriptional activation by the E2F transcription factors. The viral E1A proteins indirectly activate E2F-dependent transcription and cell cycle entry, in part, by interacting with pRb and family members to free the E2Fs. We report here that an E1A 13S isoform can unexpectedly activate E2F-responsive gene expression independently of binding to the pRb family of proteins. We demonstrate that E1A binds to E2F/DP-1 complexes through a direct interaction with DP-1. E1A appears to utilize this binding to recruit itself to E2F-regulated promoters, and this allows the E1A 13S protein, but not the E1A 12S protein, to activate transcription independently of interaction with pRb. Importantly, expression of E1A 13S, but not E1A 12S, led to significant enhancement of E2F4 occupancy of E2F sites of two E2F-regulated promoters. These observations identify a novel mechanism by which adenovirus deregulates the cell cycle and suggest that E1A 13S may selectively activate a subset of E2F-regulated cellular genes during infection.

The NS1 protein of influenza A virus interacts with heat shock protein Hsp90 in human alveolar basal epithelial cells: implication for virus-induced apoptosis

Background

Our previous study showed that the NS1 protein of highly pathogenic avian influenza A virus H5N1 induced caspase-dependent apoptosis in human alveolar basal epithelial cells (A549), supporting its function as a proapoptotic factor during viral infection, but the mechanism is still unknown.

Results

To characterize the mechanism of NS1-induced apoptosis, we used a two-hybrid system to isolate the potential NS1-interacting partners in A549 cells. We found that heat shock protein 90 (Hsp90) was able to interact with the NS1 proteins derived from both H5N1 and H3N2 viruses, which was verified by co-immunoprecitation assays. Significantly, the NS1 expression in the A549 cells dramatically weakened the interaction between Apaf-1 and Hsp90 but enhanced its interaction with cytochrome c (Cyt c), suggesting that the competitive binding of NS1 to Hsp90 might promote the Apaf-1 to associate with Cyt c and thus facilitate the activation of caspase 9 and caspase 3.

Conclusions

The present results demonstrate that NS1 protein of Influenza A Virus interacts with heat hock protein Hsp90 and meidates the apoptosis induced by influenza A virus through the caspase cascade.

EAPP: gatekeeper at the crossroad of apoptosis and p21-mediated cell-cycle arrest

We previously identified and characterized E2F-associated phospho-protein (EAPP), a nuclear phosphoprotein that interacts with the activating members of the E2F transcription factor family. EAPP levels are frequently elevated in transformed human cells. To examine the biological relevance of EAPP, we studied its properties in stressed and unstressed cells. Overexpression of EAPP in U2OS cells increased the fraction of G1 cells and lead to heightened resistance against DNA damage- or E2F1-induced apoptosis in a p21-dependent manner. EAPP itself becomes upregulated in confluent cells and after DNA damage and stimulates the expression of p21 independently of p53. It binds to the p21 promoter and seems to be required for the assembly of the transcription initiation complex. RNAi-mediated knockdown of EAPP expression brought about increased sensitivity towards DNA damage and resulted in apoptosis even in the absence of stress. Our results indicate that the level of EAPP is critical for cellular homeostasis. Too much of it results in G1 arrest and resistance to apoptosis, which, paradoxically, might favor cellular transformation. Too little EAPP seems to retard the expression not only of the p21 gene, but also of a number of other genes and ultimately results in apoptosis.

Knockdown of cyclin-dependent kinase inhibitors induces cardiomyocyte re-entry in the cell cycle

Proliferation of mammalian cardiomyocytes stops rapidly after birth and injured hearts do not regenerate adequately. High cyclin-dependent kinase inhibitor (CKI) levels have been observed in cardiomyocytes, but their role in maintaining cardiomyocytes in a post-mitotic state is still unknown. In this report, it was investigated whether CKI knockdown by RNA interference induced cardiomyocyte proliferation. We found that triple transfection with p21(Waf1), p27(Kip1), and p57(Kip2) siRNAs induced both neonatal and adult cardiomyocyte to enter S phase and increased the nuclei/cardiomyocyte ratio; furthermore, a subpopulation of cardiomyocytes progressed beyond karyokynesis, as assessed by the detection of mid-body structures and by straight cardiomyocyte counting. Intriguingly, cardiomyocyte proliferation occurred in the absence of overt DNA damage and aberrant mitotic figures. Finally, CKI knockdown and DNA synthesis reactivation correlated with a dramatic change in adult cardiomyocyte morphology that may be a prerequisite for cell division. In conclusion, CKI expression plays an active role in maintaining cardiomyocyte withdrawal from the cell cycle.

Small molecule regulators of Rb-E2F pathway as modulators of transcription

The retinoblastoma tumor suppressor protein, Rb, plays a major role in the regulation of mammalian cell cycle progression. It has been shown that Rb function is essential for the proper modulation of G1/S transition and inactivation of Rb contributes to deregulated cell proliferation. Rb exerts its cell cycle regulatory functions mainly by targeting the E2F family of transcription factors and Rb has been shown to physically interact with E2Fs 1, 2 and 3, repressing their transcriptional activity. Multiple genes involved in DNA synthesis and cell cycle progression are regulated by E2Fs, and Rb prevents their expression by inhibiting E2F activity, inducing growth arrest. It has been established that inactivation of Rb by phosphorylation, mutation, or by the interaction of viral oncoproteins leads to a release of the repression of E2F activity, facilitating cell cycle progression. Rb-mediated repression of E2F activity involves the recruitment of a variety of transcriptional co-repressors and chromatin remodeling proteins, including histone deacetylases, DNA methyltransferases and Brg1/Brm chromatin remodeling proteins. Inactivation of Rb by sequential phosphorylation events during cell cycle progression leads to a dissociation of these co-repressors from Rb, facilitating transcription. It has been found that small molecules that prevent the phosphorylation of Rb prevent the dissociation of certain co-repressors from Rb, especially Brg1, leading to the maintenance of Rb-mediated transcriptional repression and cell cycle arrest. Such small molecules have anti-cancer activities and will also act as valuable probes to study chromatin remodeling and transcriptional regulation.

p38 phosphorylates Rb on Ser567 by a novel, cell cycle-independent mechanism that triggers Rb-Hdm2 interaction and apoptosis

The retinoblastoma protein (Rb) inhibits both cell division and apoptosis, but the mechanism by which Rb alternatively regulates these divergent outcomes remains poorly understood. Cyclin dependent kinases (Cdks) promote cell division by phosphorylating and reversibly inactivating Rb by a hierarchical series of phosphorylation events and sequential conformational changes. The stress-regulated mitogen activated protein kinase (MAPK) p38 also phosphorylates Rb, but it does so in a cell cycle-independent manner that is associated with apoptosis rather than with cell division. Here, we show that p38 phosphorylates Rb by a novel mechanism that is distinct from that of Cdks. p38 bypasses the cell cycle-associated hierarchical phosphorylation and directly phosphorylates Rb on Ser567, which is not phosphorylated during the normal cell cycle. Phosphorylation by p38, but not Cdks, triggers an interaction between Rb and the human homologue of murine double minute 2 (Hdm2), leading to degradation of Rb, release of E2F1 and cell death. These findings provide a mechanistic explanation for how Rb regulates cell division and apoptosis through different kinases, and reveal how Hdm2 may functionally link the tumor suppressors Rb and p53.

E2F1 represses beta-catenin/TCF activity by direct up-regulation of Siah1

Transcription factor E2F1 is a key regulator of cell proliferation and apoptosis. Its activity is strictly controlled by the pRB/E2F pathway. In the majority of cancer cells, however, this pathway is frequently found deregulated, and the underlying mechanism involving transcriptional control by E2F1 has not yet been fully elucidated. Here we report the identification of two putative E2F1-binding sites located upstream from Siah1 transcription start site (+1). Chromatin immunoprecipitation assay reveals that transcription factor E2F1 is capable of binding to the putative sites, and luciferase reporter assay shows that E2F1 can activate transcription from the Siah1 promoter. Ectopic expression of E2F1 elevates the Siah1 level, hence suppressing the beta-catenin/TCF activity. Consistently, knock-down of endogenous E2F1 by a shRNA strategy results in reduced expression of Siah1. Moreover, repression of beta-catenin/TCF activity by E2F1 can be attenuated by shRNA-based repression of endogenous Siah1, implying that Siah1 is a bona fide E2F1 target gene, which at least partly, mediates the suppression of beta-catenin/TCF signalling pathway.

Aldose reductase regulates vascular smooth muscle cell proliferation by modulating G1/S phase transition of cell cycle

Abnormal proliferation of vascular smooth muscle cells (VSMC) is a key feature of development of cardiovascular complications, atherosclerosis, and restenosis. Patients with diabetes have higher risk for restenosis after coronary angioplasty than nondiabetic patients due to hyperglycemia-induced release of cytokines such as TNF-alpha. However, the molecular mechanisms regulating VSMC proliferation remain unclear. Herein, we report that inhibition of the polyol pathway enzyme aldose reductase (AR) prevents high glucose (HG)- and/or TNF-alpha-induced VSMC proliferation by accumulating cells at the G1 phase of the cell cycle. Treatment of VSMC with AR inhibitor sorbinil prevented HG- as well as TNF-alpha-induced phosphorylation of retinoblastoma protein and activation of E2F-1. Inhibition of AR also prevented HG- and TNF-alpha-induced phosphorylation of cyclin-dependent kinase (cdk)-2 and expression of G1/S transition regulatory proteins such as cyclin D1, cyclin E, cdk-4, c-myc, and proliferative cell nuclear antigen. More importantly, inhibition of AR prevented the increased expression of E2F-1 and proliferative cell nuclear antigen in diabetic rat aorta. Treatment of VSMC with the most abundant and toxic lipid aldehyde 4-hydroxy-trans-2-nonenal (HNE) or its glutathione conjugate [glutathionyl (GS)-HNE] or AR-catalyzed product of GS-HNE, GS-1,4-dihydroxynonane, resulted in increased E2F-1 expression. Inhibition of AR prevented HNE- or GS-HNE-induced but not GS-1,4-dihydroxynonane-induced up-regulation of E2F-1. Collectively, these results show that AR could regulate HG- and TNF-alpha-induced VSMC proliferation by altering the activation of G1/S-phase proteins such as E2F-1, cdks, and cyclins. Thus, inhibition of AR may be a useful therapeutic approach in preventing vascular complications.

Highly pathogenic avian influenza A virus H5N1 NS1 protein induces caspase-dependent apoptosis in human alveolar basal epithelial cells

Background

It is widely considered that the multifunctional NS1 protein of influenza A viruses contributes significantly disease pathogenesis by modulating a number of virus and host-cell processes, but it is highly controversial whether this non-structural protein is a proapoptotic or antiapoptotic factor in infected cells.

Results

NS1 protein of influenza A/chicken/Jilin/2003 virus, a highly pathogenic H5N1 strain, could induce apoptosis in the carcinomic human alveolar basal epithelial cells (A549) by electron microscopic and flow cytometric analyses. NS1 protein-triggered apoptosis in A549 cells is via caspase-dependent pathway.

Conclusions

Influenza A virus NS1 protein serves as a strong inducer of apoptosis in infected human respiratory epithelial cells and plays a critical role in disease pathogenesis.

E2F-1 has dual roles depending on the cell cycle

The E2F family of transcription factors play a critical role in the control of cell proliferation. E2F-1 is the major cellular target of pRB and is regulated by pRB during cell proliferation. E2F-1-mediated activation and repression of target genes occurs in different settings. The role of E2F-1 and E2F-1/pRB complexes in regulation of different target genes, and in cycling versus quiescent cells, is unclear. In this study, effects of free E2F-1 (doesn't complex with pRb) and E2F-1/pRb complex, on E2F-1 target gene expression were compared in different cell growth conditions. Findings suggest that E2F-1 acts in different ways, not only depending on the target gene but also depending on different stages of the cell cycle. For example, E2F-1 acts as part of the repression complex with pRB in the expression of DHFR, b-myb, TK and cdc2 in asynchronously growing cells; on the other hand, E2F-1 acts as an activator in the expression of the same genes in cells that are re-entering the cycle.

Antisense MDM2 enhances E2F1-induced apoptosis and the combination sensitizes androgen-sensitive [corrected] and androgen-insensitive [corrected] prostate cancer cells to radiation

We have previously shown in separate studies that MDM2 knockdown via antisense MDM2 (AS-MDM2) and E2F1 overexpression via adenoviral-mediated E2F1 (Ad-E2F1) sensitized prostate cancer cells to radiation. Because E2F1 and MDM2 affect apoptosis through both common and independent pathways, we hypothesized that coupling these two treatments would result in increased killing of prostate cancer cells. In this study, the effect of Ad-E2F1 and AS-MDM2 in combination with radiation was investigated in three prostate cancer cell lines: LNCaP cells, LNCaP-Res cells [androgen insensitive with functional p53 and androgen receptor (AR)], and PC3 cells (androgen insensitive, p53(null), and AR(null)). A supra-additive radiosensitizing effect was observed in terms of clonogenic inhibition and induction of apoptosis (caspase-3 + caspase-7 activity) in response to Ad-E2F1 plus AS-MDM2 treatments in all three cell lines. In LNCaP and LNCaP-Res, these combination treatments elevated the levels of phospho-Ser(15) p53 with significant induction of p21(waf1/cip1), phospho-gammaH2AX, PUMA, and Bax levels and reduction of AR and bcl-2 expression. Similarly, AR(null) and p53(null) PC-3 cells showed elevated levels of Bax and phospho-gammaH2AX expression. These findings show that the combination of Ad-E2F1 and AS-MDM2 significantly increases cell death in prostate cancer cells exposed to radiation and that this effect occurs in the presence or absence of AR and p53.

Balancing the decision of cell proliferation and cell fate

The control of cellular proliferation is key in the proper development of a complex organism, the maintenance of tissue homeostasis and the ability to respond to various hormonal and other inducers. Key in the control of proliferation is the retinoblastoma (Rb) protein which regulates the activity of a family of transcription factors known as E2Fs. The E2F proteins are now recognized to regulate the expression of a large number of genes associated with cell proliferation including genes encoding DNA replication as well as mitotic activities. What has also become clear over the past several years is the intimate relationship between the control of cell proliferation and the control of cell fate, particularly the activation of apoptotic pathways. Central in this connection is the Rb/E2F pathway that not only provides the primary signals for proliferation but at the same time, connects with the p53-dependent apoptotic pathway. This review addresses this inter-connection and the molecular mechanisms that control the decision between proliferation and cell death.

Avian influenza virus A/HK/483/97(H5N1) NS1 protein induces apoptosis in human airway epithelial cells

Avian H5N1 influenza virus causes a remarkably severe disease in humans, with an overall case fatality rate of greater than 50%. Human influenza A viruses induce apoptosis in infected cells, which can lead to organ dysfunction. To verify the role of H5N1-encoded NS1 in inducing apoptosis, the NS1 gene was cloned and expressed in human airway epithelial cells (NCI-H292 cells). The apoptotic events posttransfection were examined by a terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick-end-labeling assay, flow cytometric measurement of propidium iodide, annexin V staining, and Western blot analyses with antibodies specific for proapoptotic and antiapoptotic proteins. We demonstrated that the expression of H5N1 NS1 protein in NCI-H292 cells was sufficient to induce apoptotic cell death. Western blot analyses also showed that there was prominent cleavage of poly(ADP-ribose) polymerase and activation of caspase-3, caspase-7, and caspase-8 during the NS1-induced apoptosis. The results of caspase inhibitor assays further confirmed the involvement of caspase-dependent pathways in the NS1-induced apoptosis. Interestingly, the ability of H5N1 NS1 protein to induce apoptosis was much enhanced in cells pretreated with Fas ligand (the time posttransfection required to reach >30% apoptosis was reduced from 24 to 6 h). Furthermore, 24 h posttransfection, an increase in Fas ligand mRNA expression of about 5.6-fold was detected in cells transfected with H5N1 NS1. In conclusion, we demonstrated that the NS1 protein encoded by avian influenza A virus H5N1 induced apoptosis in human lung epithelial cells, mainly via the caspase-dependent pathway, which encourages further investigation into the potential for the NS1 protein to be a novel therapeutic target.

An E2F1-dependent gene expression program that determines the balance between proliferation and cell death

The Rb/E2F pathway regulates the expression of genes essential for cell proliferation but that also trigger apoptosis. During normal proliferation, PI3K/Akt signaling blocks E2F1-induced apoptosis, thus serving to balance proliferation and death. We now identify a subset of E2F1 target genes that are specifically repressed by PI3K/Akt signaling, thus distinguishing the E2F1 proliferative or apoptotic function. RNAi-mediated inhibition of several of these PI3K-repressed E2F1 target genes, including AMPK alpha 2, impairs apoptotic induction by E2F1. Activation of AMPK alpha 2 with an AMP analog further stimulates E2F1-induced apoptosis. We also show that the presence of the E2F1 apoptotic expression program in breast and ovarian tumors coincides with good prognosis, emphasizing the importance of the balance in the E2F1 proliferation/apoptotic program.

RhoBTB2 (DBC2) is a mitotic E2F1 target gene with a novel role in apoptosis

We have identified the RhoBTB2 putative tumor suppressor gene as a direct target of the E2F1 transcription factor. Overexpression of E2F1 led to up-regulation of RhoBTB2 at the level of mRNA and protein. This also occurred during the induction of E2F1 activity in the presence of cycloheximide, thus indicating that RhoBTB2 is a direct target. RNAi-mediated knockdown of E2F1 resulted in decreased RhoBTB2 protein expression, demonstrating that RhoBTB2 is a physiological target of E2F1. Because E2F1 primarily serves to transcribe genes involved in cell cycle progression and apoptosis, we explored whether RhoBTB2 played roles in either of these processes. We found RhoBTB2 expression highly up-regulated during mitosis, which was partially dependent on the presence of E2F1. Furthermore, overexpression of RhoBTB2 induced a short term increase in cell cycle progression and proliferation, while long term expression had a negative effect on these processes. We similarly found RhoBTB2 up-regulated during drug-induced apoptosis, with this being primarily dependent on E2F1. Finally, we observed that knockdown of RhoBTB2 levels via siRNA delayed the onset of drug-induced apoptosis. Collectively, we describe RhoBTB2 as a novel direct target of E2F1 with roles in cell cycle and apoptosis.

Life and death decisions by the E2F transcription factors

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

Proapoptotic role of novel gene-expression factors

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

Bcl-2 is the target of a UV-inducible apoptosis switch and a node for UV signaling

Sunlight's UVB radiation triggers cell signaling at multiple sites to induce apoptosis. The integration of these signal entry sites is not understood. Here we show that P53 and E2f1 constitute a UV-inducible apoptosis switch. At low-UV doses, wild-type cells resemble the OFF state of an siP53-treated cell, whereas at high-UV doses, the apoptosis frequency transitions to the fully ON behavior of an siE2f1-treated cell. The switch's target is Bcl-2: Rapid Bcl-2 down-regulation in response to UVB-induced DNA photoproducts is lost in P53-deficient cells, but, as for apoptosis, is restored when both P53 and its inhibited target E2f1 are absent. P53's down-regulation of Bcl-2 is mediated entirely through E2f1. Bcl-2 is also down-regulated by a separate pathway triggered by DNA photoproducts in the absence of P53 and E2f1. Four UV pathways terminating on Bcl-2 contribute to apoptosis after UVB irradiation. The apoptosis lost in p53(-/-) is completely restored by siBcl-2, implying that Bcl-2 is a rate-limiting member of this network. These results identify Bcl-2 as an integrator of several UV-induced proapoptotic signals and show that it, in turn, suppresses a direct UV-apoptosis pathway. UV-induced apoptosis requires both UV activation of the direct pathway and a separate UV disinhibition of this pathway through P53-E2f1-Bcl-2.

Functional identification of Api5 as a suppressor of E2F-dependent apoptosis in vivo

Retinoblastoma protein and E2-promoter binding factor (E2F) family members are important regulators of G1-S phase progression. Deregulated E2F also sensitizes cells to apoptosis, but this aspect of E2F function is poorly understood. Studies of E2F-induced apoptosis have mostly been carried out in tissue culture cells, and the analysis of the factors that are important for this process has been restricted to the testing of a few candidate genes. Using Drosophila as a model system, we have generated tools that allow genetic modifiers of E2F-dependent apoptosis to be identified in vivo and developed assays that allow effects on E2F-induced apoptosis to be studied in cultured cells. Genetic interactions show that dE2F1-dependent apoptosis in vivo involves dArk/Apaf1 apoptosome-dependent activation of both initiator and effector caspases and is sensitive to levels of Drosophila inhibitor of apoptosis-1 (dIAP1). Using these approaches, we report the surprising finding that apoptosis inhibitor-5/antiapoptosis clone-11 (Api5/Aac11) is a critical determinant of dE2F1-induced apoptosis in vivo and in vitro. This functional interaction occurs in multiple tissues, is specific to E2F-induced apoptosis, and is conserved from flies to humans. Interestingly, Api5/Aac11 acts downstream of E2F and suppresses E2F-dependent apoptosis without generally blocking E2F-dependent transcription. Api5/Aac11 expression is often upregulated in tumor cells, particularly in metastatic cells. We find that depletion of Api5 is tumor cell lethal. The strong genetic interaction between E2F and Api5/Aac11 suggests that elevated levels of Api5 may be selected during tumorigenesis to allow cells with deregulated E2F activity to survive under suboptimal conditions. Therefore, inhibition of Api5 function might offer a possible mechanism for antitumor exploitation.

The retinoblastoma protein (pRB) was the first human tumor suppressor to be described, and it works by limiting the activity of the E2F transcription factor. The pRB pathway is inactivated in most forms of cancer, and, accordingly, most tumor cells have deregulated E2F. Uncontrolled E2F drives cell proliferation, but it also sensitizes cells to die (apoptosis). E2F-induced apoptosis is not well understood, but it affects the development of cancer and, potentially, could be exploited for cancer treatment. To date, however, there have been very few studies of E2F-induced apoptosis in animal models. The authors describe a series of genetic tools that allow systematic studies of E2F-induced apoptosis in Drosophila. As validation, this approach identified some known regulators of E2F-dependent apoptosis and also identified Api5, a little-studied gene that had not previously been linked to E2F, as a potent suppressor of E2F-induced cell death. The effects of Api5 on E2F occur in several different tissues and are conserved from flies to humans. This last point is significant since Api5 is upregulated in cancer cells. The discovery of the E2F–Api5 interaction demonstrates that important modulators of E2F-induced apoptosis are waiting to be discovered and that they can be found using Drosophila.

DNA-binding independent cell death from a minimal proapoptotic region of E2F-1

The ability to induce cell cycle progression while evading cell death is a defining characteristic of cancer. Deregulation of E2F is a common event in most human cancers. Paradoxically, this can lead to both cell cycle progression and apoptosis. Although the way in which E2F causes cell cycle progression is well characterized, the pathways by which E2F induces cell death are less well defined. Many of the known mechanisms through which E2F induces apoptosis occur through regulation of E2F target genes. However, mutants of E2F-1 that lack the transactivation domain are still able to induce cell death. To further investigate this activity, we refined a transactivation independent mutant to identify a minimal apoptotic domain. This revealed that only 75 amino acids from within the DNA-binding domain of E2F-1 is sufficient for cell death and that this activity is also present in the DNA-binding domains of E2F-2 and E2F-3. However, analysis of this domain from E2F-1 revealed it does not bind DNA and is consequently unable to transactivate, repress or de-repress E2F target genes. This provocative observation therefore defines a potential new mechanism of death from E2F and opens up new opportunities for inducing cell death in tumours for therapeutic gain.

Jab1 is a specificity factor for E2F1-induced apoptosis

The members of the E2F family of transcription factors are key regulators of genes involved in cell cycle progression, cell fate determination, DNA damage repair, and apoptosis. Many cell-based experiments suggest that E2F1 is a stronger inducer of apoptosis than the other E2Fs. Our previous work identified the E2F1 marked box and flanking region as critical for the specificity in E2F1 apoptosis induction. We have now used a yeast two-hybrid screen to identify proteins that bind the E2F1 marked box and flanking regions, with a potential role in E2F1 apoptosis induction. We identified Jab1 as an E2F1-specific binding protein and showed that Jab1 and E2F1 coexpression synergistically induce apoptosis, coincident with an induction of p53 protein accumulation. In contrast, Jab1 does not synergize with E2F1 to promote cell cycle entry. Cells depleted of Jab1 are deficient for both E2F1-induced apoptosis and induction of p53 accumulation. We suggest that Jab1 is an essential cofactor for the apoptotic function of E2F1.

Death-signal-induced relocalization of cyclin-dependent kinase 11 to mitochondria

Fas receptor-Fas ligand interaction appears to be important in carcinogenesis, tumour outgrowth and metastasis. Emerging evidence suggests that CDK11 (cyclin-dependent kinase 11) plays a role in apoptosis and melanoma development. Here, we show that CDK11p110 protein kinase was cleaved after induction of apoptosis by Fas. The N-terminal portion of CDK11p110, CDK11p60, was translocated from the nucleus to the mitochondria. The targeting of CDK11p60 to mitochondria occurred as early as 12 h after treatment. Overexpression of EGFP (enhanced green fluorescent protein)-tagged CDK11p60 could partially break down the mitochondrial membrane potential, induce cytochrome c release and promote apoptosis. Reduction of endogenous CDK11p110 protein levels with siRNA (small interfering RNA) resulted in the suppression of both cytochrome c release and apoptosis. In addition, subcellular fractionation studies of Fas-mediated apoptosis demonstrated that CDK11p60 was associated with the mitochondrial import motor, mitochondrial heat shock protein 70. Taken together, our data suggest that CDK11p60 can contribute to apoptosis by direct signalling at the mitochondria, thereby amplifying Fas-induced apoptosis in melanoma cells.

Identification of novel E2F1 target genes regulated in cell cycle-dependent and independent manners

The transcription factor E2F mediates cell cycle-dependent expression of genes important for cell proliferation in response to growth stimulation. To further understand the role of E2F, we utilized a sensitive subtraction method to explore new E2F1 targets, which are expressed at low levels and might have been unrecognized in previous studies. We identified 33 new E2F1-inducible genes, including checkpoint genes Claspin and Rad51ap1, and four genes with unknown function required for cell cycle progression. Moreover, we found three groups of E2F1-inducible genes that were not induced by growth stimulation. At least, two groups of genes were directly induced by E2F1, indicating that E2F1 can regulate expression of genes not induced during the cell cycle. One included Neogenin, WASF1 and SGEF genes, which may have a role in differentiation or development. The other was the cyclin-dependent kinase inhibitor p27(Kip1), which was involved in suppression of inappropriate cell cycle progression induced by deregulated E2F. E2F1-responsive regions of these genes were located more upstream than those of typical E2F targets and did not have typical E2F sites. These results indicate that there are groups of E2F1 targets, which are regulated in a distinct manner from that of typical E2F targets.

Distinctions in the specificity of E2F function revealed by gene expression signatures

The E2F family of transcription factors provides essential activities for coordinating the control of cellular proliferation and cell fate. Both E2F1 and E2F3 proteins have been shown to be particularly important for cell proliferation, whereas the E2F1 protein has the capacity to promote apoptosis. To explore the basis for this specificity of function, we used DNA microarray analysis to probe for the distinctions in the two E2F activities. Gene expression profiles that distinguish either E2F1- or E2F3-expressing cells from quiescent cells are enriched in genes encoding cell cycle and DNA replication activities, consistent with many past studies. E2F1 profile is also enriched in genes known to function in apoptosis. We also identified patterns of gene expression that specifically differentiate the activity of E2F1 and E2F3; this profile is enriched in genes known to function in mitosis. The specificity of E2F function has been attributed to protein interactions mediated by the marked box domain, and we now show that chimeric E2F proteins generate expression signatures that reflect the origin of the marked box, thus linking the biochemical mechanism for specificity of function with specificity of gene activation.

Adenoviral-E2F-1 radiosensitizes p53wild-type and p53null human prostate cancer cells

PURPOSE

E2F-1 is a transcription factor that enhances the radiosensitivity of various cell lines by inducing apoptosis. However, there are conflicting data concerning whether this enhancement is mediated via p53 dependent pathways. Additionally, the role of E2F-1 in the response of human prostate cancer to radiation has not been well characterized. In this study, we investigated the effect of Adenoviral-E2F-1 (Ad-E2F-1) on the radiosensitivity of p53wild-type (LNCaP) and p53null (PC3) prostate cancer cell lines.

METHODS AND MATERIALS

LNCaP and PC3 cells were transduced with Ad-E2F-1, Adenoviral-Luciferase (Ad-Luc) control vector, or Adenoviral-p53 (Ad-p53). Expression of E2F-1 and p53 was examined by Western blot analysis. Annexin V and caspase 3 + 7 assays were performed to estimate the levels of apoptosis. Clonogenic survival assays were used to determine overall cell death. Statistical significance was determined by analysis of variance, using the Bonferroni method to correct for multiple comparisons.

RESULTS

Western blot analysis confirmed the efficacy of transductions with Ad-E2F-1 and Ad-p53. Ad-E2F-1 transduction significantly enhanced apoptosis and decreased clonogenic survival in both cell lines. These effects were compounded by the addition of RT. Although E2F-1-mediated radiosensitization was independent of p53 status, this effect was more pronounced in p53wild-type LNCaP cells. When PC3 cells were treated with Ad-p53 in combination with RT and Ad-E2F-1, there was at least an additive reduction in clonogenic survival.

CONCLUSIONS

Our results suggest that Ad-E2F-1 significantly enhances the response of p53wild-type and p53null prostate cancer cells to radiation therapy, although radiosensitization is more pronounced in the presence of p53. Ad-E2F-1 may be a useful adjunct to radiation therapy in the treatment of prostate cancer.