Jab1 is a specificity factor for E2F1-induced apoptosis

Identification of COP9 signalosome (CSN) subunits and antiviral function analysis of CSN5 in shrimp

The constitutive photomorphogenesis 9 (COP9) signalosome (CSN) typically composing of eight subunits (CSN1-8) mediates the process of deneddylation and deubiquitination. The fifth subunit of COP9 signalosome, CSN5, has special characteristics compared with the other seven subunits, and plays vital roles in the deneddylation activity and diverse cellular processes. However, the role of CSN5 in antiviral immunity is not clear. In this study, we identified 8 subunits (CSN1-8) of COP9 signalosome in shrimp Marsupenaeus japonicus. CSN1-6 were existed in all tested tissues, but CSN7-CSN8 were not detected in hepatopancreas. After WSSV challenged, the expression level of Csn1 to Csn4, and Csn6 to Csn8 were highly decreased, but the expression level of Csn5 was conspicuously increased in shrimp challenged by white spot syndrome virus (WSSV). The CSN5 was recombinantly expressed in Escherichia coli and its polyclonal antibody was prepared. The expression level of CSN5 was conspicuously increased at RNA and protein levels in the shrimp challenged by WSSV. After knockdown of Csn5 by RNA interference, the WSSV replication was obviously increased in shrimp. When injected the recombinant protein of CSN5 with the membrane penetrating peptide into shrimp, WSSV replication was inhibited and the survival rate of shrimp was significantly improved compared with control. We further analyzed the expression of antimicrobial peptides (AMPs) in Csn5-RNAi shrimp, and the results showed that the expression of several AMPs was declined significantly. These results indicate that CSN5 inhibits replication of WSSV via regulating expression of AMPs in shrimp, and the recombinant CSN5 might be used in shrimp aquaculture for the white spot syndrome disease control.

COP9 signalosome component CSN-5 stabilizes PUF proteins FBF-1 and FBF-2 in Caenorhabditis elegans germline stem and progenitor cells

RNA-binding proteins FBF-1 and FBF-2 (FBFs) are required for germline stem cell maintenance and the sperm/oocyte switch in Caenorhabditis elegans, although the mechanisms controlling FBF protein levels remain unknown. We identified an interaction between both FBFs and CSN-5), a component of the constitutive photomorphogenesis 9 (COP9) signalosome best known for its role in regulating protein degradation. Here, we find that the Mpr1/Pad1 N-terminal metalloprotease domain of CSN-5 interacts with the Pumilio and FBF RNA-binding domain of FBFs and the interaction is conserved for human homologs CSN5 and PUM1. The interaction between FBF-2 and CSN-5 can be detected in vivo by proximity ligation. csn-5 mutation results in the destabilization of FBF proteins, which may explain previously observed decrease in the numbers of germline stem and progenitor cells, and disruption of oogenesis. The loss of csn-5 does not decrease the levels of a related PUF protein PUF-3, and csn-5(lf) phenotype is not enhanced by fbf-1/2 knockdown, suggesting that the effect is specific to FBFs. The effect of csn-5 on oogenesis is largely independent of the COP9 signalosome and is cell autonomous. Surprisingly, the regulation of FBF protein levels involves a combination of COP9-dependent and COP9-independent mechanisms differentially affecting FBF-1 and FBF-2. This work supports a previously unappreciated role for CSN-5 in the stabilization of germline stem cell regulatory proteins FBF-1 and FBF-2.

Post-translational modifications of lysine-specific demethylase 1

Lysine-specific demethylase 1 (LSD1) is crucial for regulating gene expression by catalyzing the demethylation of mono- and di-methylated histone H3 lysine 4 (H3K4) and lysine 9 (H3K9) and non-histone proteins through the amine oxidase activity with FAD+ as a cofactor. It interacts with several protein partners, which potentially contributes to its diverse substrate specificity. Given its pivotal role in numerous physiological and pathological conditions, the function of LSD1 is closely regulated by diverse post-translational modifications (PTMs), including phosphorylation, ubiquitination, methylation, and acetylation. In this review, we aim to provide a comprehensive understanding of the regulation and function of LSD1 following various PTMs. Specifically, we will focus on the impact of PTMs on LSD1 function in physiological and pathological contexts and discuss the potential therapeutic implications of targeting these modifications for the treatment of human diseases.

Clinical Implication of E2F Transcription Factor 1 in Hepatocellular Carcinoma Tissues

Background: To date, the clinical management of advanced hepatocellular carcinoma (HCC) patients remains challenging and the mechanisms of E2F transcription factor 1 (E2F1) underlying HCC are obscure. Materials and Methods: Our study integrated datasets mined from several public databases to comprehensively understand the deregulated expression status of E2F1. Tissue microarrays and immunohistochemistry staining was used to validate E2F1 expression level. The prognostic value of E2F1 was assessed. In-depth subgroup analyses were implemented to compare the differentially expressed levels of E2F1 in HCC patients with various tumor stages. Functional enrichments were used to address the predominant targets of E2F1 and shedding light on their potential roles in HCC. Results: We confirmed the elevated expression of E2F1 in HCC. Subgroup analyses indicated that elevated E2F1 level was independent of various stages in HCC. E2F1 possessed moderate discriminatory capability in differentiating HCC patients from non-HCC controls. Elevated E2F1 correlated with Asian race, tumor classification, neoplasm histologic grade, eastern cancer oncology group, and plasma AFP levels. Furthermore, high E2F1 correlated with poor survival condition and pooled HR signified E2F1 as a risk factor for HCC. Enrichment analysis of differentially expressed genes, coexpressed genes, and putative targets of E2F1 emphasized the importance of cell cycle pathway, where CCNE1 and CCNA2 served as hub genes. Conclusions: We confirmed the upregulation of E2F1 and explored the prognostic value of E2F1 in HCC patients. Two putative targeted genes (CCNE1 and CCNA2) of E2F1 were identified for their potential roles in regulating cell cycle and promote antiapoptotic activity in HCC patients.

Regulatory mechanisms and therapeutic potential of JAB1 in neurological development and disorders

c-Jun activation domain binding protein-1 (JAB1) is a multifunctional regulator that plays vital roles in diverse cellular processes. It regulates AP-1 transcriptional activity and also acts as the fifth component of the COP9 signalosome complex. While JAB1 is considered an oncoprotein that triggers tumor development, recent studies have shown that it also functions in neurological development and disorders. In this review, we summarize the general features of the JAB1 gene and protein, and present recent updates on the regulation of JAB1 expression. Moreover, we also highlight the functional roles and regulatory mechanisms of JAB1 in neurodevelopmental processes such as neuronal differentiation, synaptic morphogenesis, myelination, and hair cell development and in the pathogenesis of some neurological disorders such as Alzheimer's disease, multiple sclerosis, neuropathic pain, and peripheral nerve injury. Furthermore, current challenges and prospects are discussed, including updates on drug development targeting JAB1.

Evolution and Structure of API5 and Its Roles in Anti-Apoptosis

Apoptosis, also named programmed cell death, is a highly conserved physiological mechanism. Apoptosis plays crucial roles in many life processes, such as tissue development, organ formation, homeostasis maintenance, resistance against external aggression, and immune responses. Apoptosis is regulated by many genes, among which Apoptosis Inhibitor-5 (API5) is an effective inhibitor, though the structure of API5 is completely different from the other known Inhibitors of Apoptosis Proteins (IAPs). Due to its high expression in many types of tumors, API5 has received extensive attention, and may be an effective target for cancer treatment. In order to comprehensively and systematically understand the biological roles of API5, we summarized the evolution and structure of API5 and its roles in anti-apoptosis in this review.

The E2F1 transcription factor and RB tumor suppressor moonlight as DNA repair factors

The E2F1 transcription factor and RB tumor suppressor are best known for their roles in regulating the expression of genes important for cell cycle progression but, they also have transcription-independent functions that facilitate DNA repair at sites of damage. Depending on the type of DNA damage, E2F1 can recruit either the GCN5 or p300/CBP histone acetyltransferases to deposit different histone acetylation marks in flanking chromatin. At DNA double-strand breaks, E2F1 also recruits RB and the BRG1 ATPase to remodel chromatin and promote loading of the MRE11-RAD50-NBS1 complex. Knock-in mouse models demonstrate important roles for E2F1 post-translational modifications in regulating DNA repair and physiological responses to DNA damage. This review highlights how E2F1 moonlights in DNA repair, thus revealing E2F1 as a versatile protein that recruits many of the same chromatin-modifying enzymes to sites of DNA damage to promote repair that it recruits to gene promoters to regulate transcription.

COP9 signalosome complex subunit 5, an IFT20 binding partner, is essential to maintain male germ cell survival and acrosome biogenesis†

Intraflagellar transport protein 20 (IFT20) is essential for spermatogenesis in mice. We discovered that COPS5 was a major binding partner of IFT20. COPS5 is the fifth component of the constitutive photomorphogenic-9 signalosome (COP9), which is involved in protein ubiquitination and degradation. COPS5 is highly abundant in mouse testis. Mice deficiency in COPS5 specifically in male germ cells showed dramatically reduced sperm numbers and were infertile. Testis weight was about one third compared to control adult mice, and germ cells underwent significant apoptosis at a premeiotic stage. Testicular poly (ADP-ribose) polymerase-1, a protein that helps cells to maintain viability, was dramatically decreased, and Caspase-3, a critical executioner of apoptosis, was increased in the mutant mice. Expression level of FANK1, a known COPS5 binding partner, and a key germ cell apoptosis regulator was also reduced. An acrosome marker, lectin PNA, was nearly absent in the few surviving spermatids, and expression level of sperm acrosome associated 1, another acrosomal component was significantly reduced. IFT20 expression level was significantly reduced in the Cops5 knockout mice, and it was no longer present in the acrosome, but remained in the Golgi apparatus of spermatocytes. In the conditional Ift20 mutant mice, COPS5 localization and testicular expression levels were not changed. COP9 has been shown to be involved in multiple signal pathways, particularly functioning as a co-factor for protein ubiquitination. COPS5 is believed to maintain normal spermatogenesis through multiple mechanisms, including maintaining male germ cell survival and acrosome biogenesis, possibly by modulating protein ubiquitination.

Role of the COP9 Signalosome (CSN) in Cardiovascular Diseases

The constitutive photomorphogenesis 9 (COP9) signalosome (CSN) is an evolutionarily conserved multi-protein complex, consisting of eight subunits termed CSN1-CSN8. The main biochemical function of the CSN is the control of protein degradation via the ubiquitin-proteasome-system through regulation of cullin-RING E3-ligase (CRL) activity by deNEDDylation of cullins, but the CSN also serves as a docking platform for signaling proteins. The catalytic deNEDDylase (isopeptidase) activity of the complex is executed by CSN5, but only efficiently occurs in the three-dimensional architectural context of the complex. Due to its positioning in a central cellular pathway connected to cell responses such as cell-cycle, proliferation, and signaling, the CSN has been implicated in several human diseases, with most evidence available for a role in cancer. However, emerging evidence also suggests that the CSN is involved in inflammation and cardiovascular diseases. This is both due to its role in controlling CRLs, regulating components of key inflammatory pathways such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and complex-independent interactions of subunits such as CSN5 with inflammatory proteins. In this case, we summarize and discuss studies suggesting that the CSN may have a key role in cardiovascular diseases such as atherosclerosis and heart failure. We discuss the implicated molecular mechanisms ranging from inflammatory NF-κB signaling to proteotoxicity and necrosis, covering disease-relevant cell types such as myeloid and endothelial cells or cardiomyocytes. While the CSN is considered to be disease-exacerbating in most cancer entities, the cardiovascular studies suggest potent protective activities in the vasculature and heart. The underlying mechanisms and potential therapeutic avenues will be critically discussed.

CINSARC signature as a prognostic marker for clinical outcome in sarcomas and beyond

Prognostication is a key issue for sarcoma patients' care as it triggers the therapeutic approach including chemotherapy, which is still not standard for localized patients. Current prognostic evaluation, based on the FNCLCC grading system, has recently been improved by the CINSARC signature outperforming histology-based grading system by identifying high-risk patients in every grade, even in those considered as low. CINSARC is an expression-based signature related to mitosis and chromosome integrity with prognostic value in a wide range of cancers additional to sarcoma. First developed with frozen material, CINSARC is now coupled with NanoString technology allowing evaluation from FFPE blocks used in clinical practice. Consequently, CINSARC is currently evaluated in clinical trials with a dual objective of demonstrating the benefit of chemotherapy in sarcoma patients and testing its response prediction. Considering its overarching value in oncology, its development is welcome in any cancers where the prognostication needs to be improved.

The pivotal oncogenic role of Jab1/CSN5 and its therapeutic implications in human cancer

Jab1/CSN5 is a conserved multifunctional protein involved in ubiquitin-mediated protein degradation. Deregulation of Jab1/CSN5 can exert dramatic effects on diverse cellular functions, including DNA repair, cell cycle control, apoptosis, angiogenesis, and signal transduction, all of which are critical for tumor development. Although increasing evidence has demonstrated that Jab1/CSN5 was overexpressed in a variety of human cancers and usually correlated with poor prognosis, little was known about the underlying regulatory principles that coordinated its function. In this review, we highlight recent advances of the oncogenic role of Jab1/CSN5 and its potential as a therapeutic target for anticancer intervention.

The Nonsynonymous Polymorphisms Val276Met and Gly393Ser of E2F1 Gene are Strongly Associated with Lung, and Head and Neck Cancers

AIM

The early gene factor-2 (E2F), a family of transcription factors, is involved in cell cycle regulation. Deregulated expression of most of the members of the E2F family is associated with various human cancers. In this study, we investigated the association between the E2F1 genetic variants rs3213173 (C/T) (Val276Met) and rs3213176 (G/A) (Gly393Ser) with the risk of lung cancer (LC) and head and neck cancer (HNC) in 190 patients and 230 control samples.

MATERIALS AND METHODS

We used polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and mutagenic primer-based PCR-RFLP methods to genotype all target polymorphisms.

RESULTS

The rs3213173 (C/T) polymorphism was associated with LC risk in the homozygous model (odds ratio [OR] = 2.954, 95% confidence interval [CI] 1.366-6.386; p = 0.004) as well as in heterozygous model (OR = 2.314; 95% CI = 1.369-3.912; p = 0.001). A significant association was also observed for the rs3213176 (G/A) polymorphism with LC risk in homozygous model, GG versus AA (OR = 2.750; 95% CI = 1.236-6.118; p = 0.01); in heterozygous model, GG versus GA (OR = 2.111; 95% CI = 1.256-3.549; p = 0.004); and in combined mutant GG versus GA+AA (OR = 2.214; 95% CI = 1.343-3.650; p = 0.001). The rs3213176 (G/A) marker was also associated with HNC risk.

CONCLUSIONS

Our findings reveal that the rs3213173 (C/T) and rs3213176 (G/A) polymorphisms of the E2F1 gene are genetic risk factors for susceptibility to LC and HNC in the North Indian Population.

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.

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.

Roles of Multifunctional COP9 Signalosome Complex in Cell Fate and Implications for Drug Discovery

The eight subunits containing COP9 signalosome (CSN) complex, is highly conserved among eukaryotes. CSN, identified as a negative regulator of photomorphogenesis, has also been demonstrated to be important in proteolysis, cellular signal transduction and cell cycle regulation in various eukaryotic organisms. This review mainly summarizes the roles of CSN in cell cycle regulation, signal transduction and apoptosis, and its potential as diagnostic biomarkers, drug targets for cancer and infectious diseases. J. Cell. Physiol. 232: 1246-1253, 2017. © 2016 Wiley Periodicals, Inc.

Human MageB2 Protein Expression Enhances E2F Transcriptional Activity, Cell Proliferation, and Resistance to Ribotoxic Stress

MageB2 belongs to the melanoma antigen gene (MAGE-I) family of tumor-specific antigens. Expression of this gene has been detected in human tumors of different origins. However, little is known about the protein function and how its expression affects tumor cell phenotypes. In this work, we found that human MageB2 protein promotes tumor cell proliferation in a p53-independent fashion, as observed both in cultured cells and growing tumors in mice. Gene expression analysis showed that MageB2 enhances the activity of E2F transcription factors. Mechanistically, the activation of E2Fs is related to the ability of MageB2 to interact with the E2F inhibitor HDAC1. Cellular distribution of MageB2 protein includes the nucleoli. Nevertheless, ribotoxic drugs rapidly promote its nucleolar exit. We show that MageB2 counteracts E2F inhibition by ribosomal proteins independently of Mdm2 expression. Importantly, MageB2 plays a critical role in impairing cell cycle arrest in response to Actinomycin D. The data presented here support a relevant function for human MageB2 in cancer cells both under cycling and stressed conditions, presenting a distinct functional feature with respect to other characterized MAGE-I proteins.

Epigenetic factor EPC1 is a master regulator of DNA damage response by interacting with E2F1 to silence death and activate metastasis-related gene signatures

Transcription factor E2F1 is a key regulator of cell proliferation and apoptosis. Recently, it has been shown that aberrant E2F1 expression often detectable in advanced cancers contributes essentially to cancer cell propagation and characterizes the aggressive potential of a tumor. Conceptually, this requires a subset of malignant cells capable of evading apoptotic death through anticancer drugs. The molecular mechanism by which the pro-apoptotic activity of E2F1 is antagonized is widely unclear. Here we report a novel function for EPC1 (enhancer of polycomb homolog 1) in DNA damage protection. Depletion of EPC1 potentiates E2F1-mediated apoptosis in response to genotoxic treatment and abolishes tumor cell motility. We found that E2F1 directly binds to the EPC1 promoter and EPC1 vice versa physically interacts with bifunctional E2F1 to modulate its transcriptional activity in a target gene-specific manner. Remarkably, nuclear-colocalized EPC1 activates E2F1 to upregulate the expression of anti-apoptotic survival genes such as BCL-2 or Survivin/BIRC5 and inhibits death-inducing targets. The uncovered cooperativity between EPC1 and E2F1 triggers a metastasis-related gene signature in advanced cancers that predicts poor patient survival. These findings unveil a novel oncogenic function of EPC1 for inducing the switch into tumor progression-relevant gene expression that may help to set novel therapies.

E2F1-Mediated Induction of NFYB Attenuates Apoptosis via Joint Regulation of a Pro-Survival Transcriptional Program

The E2F1 transcription factor regulates cell proliferation and apoptosis through the control of a considerable variety of target genes. Previous work has detailed the role of other transcription factors in mediating the specificity of E2F function. Here we identify the NF-YB transcription factor as a novel direct E2F1 target. Genome-wide expression analysis of the effects of NFYB knockdown on E2F1-mediated transcription identified a large group of genes that are co-regulated by E2F1 and NFYB. We also provide evidence that knockdown of NFYB enhances E2F1-induced apoptosis, suggesting a pro-survival function of the NFYB/E2F1 joint transcriptional program. Bioinformatic analysis suggests that deregulation of these NFY-dependent E2F1 target genes might play a role in sarcomagenesis as well as drug resistance.

To live or let die - complexity within the E2F1 pathway

The E2F1 transcription factor is a recognized regulator of the cell cycle as well as a potent mediator of DNA damage-induced apoptosis and the checkpoint response. Understanding the diverse and seemingly dichotomous functions of E2F1 activity has been the focus of extensive ongoing research. Although the E2F pathway is frequently deregulated in cancer, the contributions of E2F1 itself to tumorigenesis, as a promoter of proliferation or cell death, are far from understood. In this review we aim to provide an update on our current understanding of E2F1, with particular insight into its novel interaction partners and post-translational modifications, as a means to explaining its diverse functional complexity.

Emerging roles of Jab1/CSN5 in DNA damage response, DNA repair, and cancer

Jab1/CSN5 is a multifunctional protein that plays an important role in integrin signaling, cell proliferation, apoptosis, and the regulation of genomic instability and DNA repair. Dysregulation of Jab1/CSN5 activity has been shown to contribute to oncogenesis by functionally inactivating several key negative regulatory proteins and tumor suppressors. In this review, we discuss our current understanding of the relationship between Jab1/CSN5 and DNA damage and summarize recent findings regarding opportunities for and challenges to therapeutic intervention.

E2f2 induces cone photoreceptor apoptosis independent of E2f1 and E2f3

The 'activating' E2fs (E2f1-3) are transcription factors that potently induce quiescent cells to divide. Work on cultured fibroblasts suggested they were essential for division, but in vivo analysis in the developing retina and other tissues disproved this notion. The retina, therefore, is an ideal location to assess other in vivo adenovirus E2 promoter binding factor (E2f) functions. It is thought that E2f1 directly induces apoptosis, whereas other activating E2fs only induce death indirectly by upregulating E2f1 expression. Indeed, mouse retinoblastoma (Rb)-null retinal neuron death requires E2f1, but not E2f2 or E2f3. However, we report an entirely distinct mechanism in dying cone photoreceptors. These neurons survive Rb loss, but undergo apoptosis in the cancer-prone retina lacking both Rb and its relative p107. We show that while E2f1 killed Rb/p107 null rod, bipolar and ganglion neurons, E2f2 was required and sufficient for cone death, independent of E2f1 and E2f3. Moreover, whereas E2f1-dependent apoptosis was p53 and p73-independent, E2f2 caused p53-dependent cone death. Our in vivo analysis of cone photoreceptors provides unequivocal proof that E2f-induces apoptosis independent of E2f1, and reveals distinct E2f1- and E2f2-activated death pathways in response to a single tumorigenic insult.

Hypoxia-inducible factor 1 promoter-induced JAB1 overexpression enhances chemotherapeutic sensitivity of lung cancer cell line A549 in an anoxic environment

The presence of lung cancer cells in anoxic zones is a key cause od chemotherapeutic resistance. Thus, it is necessary to enhance the sensitivity of such lung cancer cells. However, loss of efficient gene therapeutic targeting and inefficient objective gene expression in the anoxic zone in lung cancer are dilemmas. In the present study, a eukaryotic expression plasmid pUC57-HRE-JAB1 driven by a hypoxia response elements promoter was constructed and introduced into lung cancer cell line A549. The cells were then exposed to a chemotherapeutic drug cis-diamminedichloroplatinum (C-DDP). qRT-PCR and western blotting were used to determine the mRNA and protein level and flow cytometry to examine the cell cycle and apoptosis of A549 transfected pUC57-HRE-JAB1. The results showed that JAB1 gene in the A549 was overexpressed after the transfection, cell proliferation being arrested in G1 phase and the apoptosis ratio significantly increased. Importantly, introduction of pUC57-HRE-JAB1 significantly increased the chemotherapeutic sensitivity of A549 in an anoxic environment. In conclusion, JAB1 overexpression might provide a novel strategy to overcome chemotherapeutic resistance in lung cancer.

CSN5 specifically interacts with CDK2 and controls senescence in a cytoplasmic cyclin E-mediated manner

The fifth component (CSN5) of the mammalian COP9 signalosome complex plays an essential role in cell proliferation and senescence, but its molecular mediator remains to be determined. Here, we searched for interactors among various cell cycle regulators, and found that CSN5, but not the CSN holo-complex, bound to CDK2 in vivo and in vitro. Depletion of CSN5 enhanced phosphorylation of CDK2 by Akt, resulting in cytoplasmic accumulation of CDK2 together with cyclin E in a leptomycin B-resistant manner, and impaired phosphorylation of the retinoblastoma protein. Additional knockdown of CDK2, which reduced the expression of cyclin E to the normal level, did not restore cell proliferation, but significantly suppressed senescence in CSN5-depleted cells. Enforced expression of cytoplasmic cyclin E induced premature senescence in immortalized cell lines. These results show that CSN5 functions through CDK2 to control premature senescence in a novel way, depending on cyclin E in the cytoplasm.

NEDDylation controls the target specificity of E2F1 and apoptosis induction

The transcription factor E2F1 has pivotal roles in both cell proliferation and cell death, and is an important molecular target in cancer. Under proliferative conditions E2F1 induces the expression of genes that promote cell cycle progression, such as E2F2, whereas under proapoptotic conditions E2F1 induces expression of genes such as p73 that lead to apoptosis. The mechanism by which the apoptotic function of E2F1 is activated remains unclear, however. We now show that members of the E2F family are covalently conjugated with the ubiquitin-like modifier NEDD8. Overexpression of SENP8, a NEDD8-specific cysteine protease, resulted in deNEDDylation of E2F1 and promoted its transactivation activity at the p73 gene but not at the E2F2 gene. Knockdown of SENP8, on the other hand, attenuated p73 expression and apoptosis induced by E2F1 or by DNA damage. SENP8 also promoted the interaction between E2F1 and its cofactor Microcephalin 1, which is required for p73 induction. These results suggest that NEDDylation is a molecular trigger that modifies the target specificity of E2F1, and could have important implications for E2F1 regulation of apoptosis.

DNA damage signals through differentially modified E2F1 molecules to induce apoptosis

E2F transcription can lead to cell proliferation or apoptosis, indicating that E2Fs control opposing functions. In a similar manner, DNA double-strand breaks can signal to induce cell cycle arrest or apoptosis. Specifically, pRB is activated following DNA damage, allowing it to bind to E2Fs and block transcription at cell cycle promoters; however, E2F1 is simultaneously activated, leading to transcription at proapoptotic promoters. We examined this paradoxical control of E2F transcription by studying how E2F1's interaction with pRB is regulated following DNA damage. Our work reveals that DNA damage signals create multiple forms of E2F1 that contain mutually exclusive posttranslational modifications. Specifically, E2F1 phospho-serine 364 is found only in complex with pRB, while E2F1 phosphorylation at serine 31 and acetylation function to create a pRB-free form of E2F1. Both pRB-bound and pRB-free modifications on E2F1 are essential for the activation of TA-p73 and the maximal induction of apoptosis. Chromatin immunoprecipitation demonstrated that E2F1 phosphorylated on serine 364 is also present at proapoptotic gene promoters during the induction of apoptosis. This indicates that distinct populations of E2F1 are organized in response to DNA damage signaling. Surprisingly, these complexes act in parallel to activate transcription of proapoptotic genes. Our data suggest that DNA damage signals alter pRB and E2F1 to engage them in functions leading to apoptotic induction that are distinct from pRB-E2F regulation in cell cycle control.

Jab1/CSN5 mediates E2F dependent expression of mitotic and apoptotic but not DNA replication targets

The E2F transcription factors are critical regulators of cell cycle and cell fate control. Several classes of E2F target genes have been categorized based on their roles in DNA replication, mitosis, apoptosis, DNA repair, etc. How E2Fs coordinate the appropriate and timely expression of these functionally disparate gene products is poorly understood at a molecular level. We previously showed that the E2F1 binding partner Jab1/CSN5 promotes E2F1-dependent induction of apoptosis but not proliferation. To better understand how Jab1 regulates E2F1 dependent transcription, we performed gene expression analysis to identify E2F target genes most and least affected by shRNA depletion of Jab1. We find that a significant number of apoptotic and mitotic E2F target genes are poorly expressed in cells lacking Jab1/CSN5, whereas DNA replication genes are generally still highly expressed. Chromatin immunoprecipitation analysis indicates that both Jab1 and E2F1 co-occupy apoptotic and mitotic, but not DNA replication target genes. We explored a potential connection between PI3K activity and Jab1/E2F1 target gene induction, and found that E2F1/Jab1 co-induction of apoptotic target genes can be inhibited by activated PI3K. Furthermore, PI3K activity interferes with formation of the E2F1/Jab1 complex by co-immunoprecipitation. Jab1/CSN5 is upregulated in a variety of human tumors, but it's unclear how its pro-proliferatory and apoptotic functions are regulated in this context. We explored the link between increased Jab1 levels and PI3K function in tumors and detected a highly significant correlation between elevated Jab1/CSN5 levels and PI3K activity in breast, ovarian, lung and prostate cancers.

Cellular inhibitor of apoptosis protein-1 (cIAP1) can regulate E2F1 transcription factor-mediated control of cyclin transcription

The inhibitor of apoptosis protein cIAP1 (cellular inhibitor of apoptosis protein-1) is a potent regulator of the tumor necrosis factor (TNF) receptor family and NF-κB signaling pathways in the cytoplasm. However, in some primary cells and tumor cell lines, cIAP1 is expressed in the nucleus, and its nuclear function remains poorly understood. Here, we show that the N-terminal part of cIAP1 directly interacts with the DNA binding domain of the E2F1 transcription factor. cIAP1 dramatically increases the transcriptional activity of E2F1 on synthetic and CCNE promoters. This function is not conserved for cIAP2 and XIAP, which are cytoplasmic proteins. Chromatin immunoprecipitation experiments demonstrate that cIAP1 is recruited on E2F binding sites of the CCNE and CCNA promoters in a cell cycle- and differentiation-dependent manner. cIAP1 silencing inhibits E2F1 DNA binding and E2F1-mediated transcriptional activation of the CCNE gene. In cells that express a nuclear cIAP1 such as HeLa, THP1 cells and primary human mammary epithelial cells, down-regulation of cIAP1 inhibits cyclin E and A expression and cell proliferation. We conclude that one of the functions of cIAP1 when localized in the nucleus is to regulate E2F1 transcriptional activity.

JAB1/CSN5: a new player in cell cycle control and cancer

c-Jun activation domain-binding protein-1 (Jab1) acts as a modulator of intracellular signaling and affects cellular proliferation and apoptosis, through its existence as a monomer or as the fifth component of the constitutive photomorphogenic-9 signalosome (CSN5). Jab1/CSN5 is involved in transcription factor specificity, deneddylation of NEDD8, and nuclear-to-cytoplasmic shuttling of key molecules. Jab1/CSN5 activities positively and negatively affect a number of pathways, including integrin signaling, cell cycle control, and apoptosis. Also, more recent studies have demonstrated the intriguing roles of Jab1/CSN5 in regulating genomic instability and DNA repair. The effects of Jab1/CSN5's multiple protein interactions are generally oncogenic in nature, and overexpression of Jab1/CSN5 in cancer provides evidence that it is involved in the tumorigenic process. In this review, we highlight our current knowledge of Jab1/CSN5 function and the recent discoveries in dissecting the Jab1 signaling pathway. Further, we also discuss the regulation of Jab1/CSN5 in cancers and its potential as a therapeutic target.

Regulation of E2F1-induced apoptosis by the nucleolar protein RRP1B

Regulation of the E2F family of transcription factors is important in control of cellular proliferation; dysregulation of the E2Fs is a hallmark of many cancers. One member of the E2F family, E2F1, also has the paradoxical ability to induce apoptosis; however, the mechanisms underlying this selectivity are not fully understood. We now identify a nucleolar protein, RRP1B, as an E2F1-specific transcriptional target. We characterize the RRP1B promoter and demonstrate its selective response to E2F1. Consistent with the activation of E2F1 activity upon DNA damage, RRP1B is induced by several DNA-damaging agents. Importantly, RRP1B is required for the expression of certain E2F1 proapoptotic target genes and the induction of apoptosis by DNA-damaging agents. This activity is mediated in part by complex formation between RRP1B and E2F1 on selective E2F1 target gene promoters. Interaction between RRP1B and E2F1 can be found inside the nucleolus and diffuse nucleoplasmic punctates. Thus, E2F1 makes use of its transcriptional target RRP1B to activate other genes directly involved in apoptosis. Our data also suggest an underappreciated role for nucleolar proteins in transcriptional regulation.

E2F1 mediates DNA damage and apoptosis through HCF-1 and the MLL family of histone methyltransferases

E2F1 is a key positive regulator of human cell proliferation and its activity is altered in essentially all human cancers. Deregulation of E2F1 leads to oncogenic DNA damage and anti-oncogenic apoptosis. The molecular mechanisms by which E2F1 mediates these two processes are poorly understood but are important for understanding cancer progression. During the G1-to-S phase transition, E2F1 associates through a short DHQY sequence with the cell-cycle regulator HCF-1 together with the mixed-lineage leukaemia (MLL) family of histone H3 lysine 4 (H3K4) methyltransferases. We show here that the DHQY HCF-1-binding sequence permits E2F1 to stimulate both DNA damage and apoptosis, and that HCF-1 and the MLL family of H3K4 methyltransferases have important functions in these processes. Thus, HCF-1 has a broader role in E2F1 function than appreciated earlier. Indeed, sequence changes in the E2F1 HCF-1-binding site can modulate both up and down the ability of E2F1 to induce apoptosis indicating that HCF-1 association with E2F1 is a regulator of E2F1-induced apoptosis.

A combinatorial mechanism for determining the specificity of E2F activation and repression

Various studies have detailed the role of E2F proteins in both transcription activation and repression. Further work has shown that distinct promoter elements, but comprising the same E2F recognition motif, confer positive or negative E2F control and that this reflects binding of either activator or repressor E2F proteins respectively. We now show that the specificity of binding of an activator or repressor E2F protein is determined by adjacent sequences that bind a cooperating transcription factor. We propose that the functional E2F element is a module comprising not only the E2F binding site but also the adjacent site for the cooperating transcription factor.

The E2F family and the role of E2F1 in apoptosis

The E2F family of transcription factors plays a pivotal role in the regulation of cellular proliferation and differentiation. Although the deregulation of E2Fs is considered an oncogenic event that predisposes immortalized cells to transformation, paradoxically, E2F1 is also equipped with an ability to induce apoptosis under certain cellular contexts. It has become evident that E2Fs, in particular E2F1, participate in many aspects of the apoptotic process, either by acting alone or in cooperation with other factors, such as p53, to protect organisms from tumor development in the face of oncogenic lesions. Given the frequent inactivation of p53 in human cancers, the E2F1-induced apoptosis pathway is rapidly gaining attention as a key mechanism to compensate the loss of p53 in human tumors. In this review, we will focus on the recent progress in our understanding of E2F1-mediated apoptosis and discuss how these discoveries can be translated into potential therapeutic intervention.

COP9-associated CSN5 regulates exosomal protein deubiquitination and sorting

Ubiquitinated endosomal proteins that are deposited into the lumens of multivesicular bodies are either sorted for lysosomal-mediated degradation or secreted as exosomes into the extracellular milieu. The mechanisms that underlie the sorting of cellular cargo proteins are currently unknown. In this study, we show that the COP9 signalosome (CSN)-associated protein CSN5 quantitatively regulated proteins that were sorted into exosomes. Western blot analysis of exosomal proteins indicated that small interfering (si)RNA knockdown of CSN5 results in increased levels of both ubiquitinated and non-ubiquitinated exosomal proteins, including heat shock protein 70, in comparison with exosomes isolated from the supernatants of 293 cells transfected with scrambled siRNA. Furthermore, 293 cells transfected with JAB1/MPN/Mov34 metalloenzyme domain-deleted CSN5 produced exosomes with higher levels of ubiquitinated heat shock protein 70, which did not affect non-ubiquitinated heat shock protein 70 levels. The loss of COP9-associated deubiquitin activity of CSN5 also led to the enhancement of HIV Gag that was sorted into exosomes as well as the promotion of HIV-1 release, suggesting that COP9-associated CSN5 regulates the sorting of a number of exosomal proteins in both a CSN5 JAB1/MPN/Mov34 metalloenzyme domain-dependent and -independent manner. We propose that COP9-associated CSN5 regulates exosomal protein sorting in both a deubiquitinating activity-dependent and -independent manner, which is contrary to the current idea of ubiquitin-dependent sorting of proteins to exosomes.

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.

E2F - at the crossroads of life and death

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

Stable form of JAB1 enhances proliferation and maintenance of hematopoietic progenitors

Overexpression of JAB1 is observed in a variety of human cancers, but how JAB1 is involved in tumor development remained to be investigated. Here we analyzed mice with modified Jab1 expression. Mice ectopically expressing a more stable form of JAB1 protein under the control of a constitutive promoter were rescued from the embryonic lethality caused by the Jab1(-/-) allele and developed a myeloproliferative disorder in a gene dosage-dependent manner. Hematopoietic cells from the bone marrow of Jab1 transgenic mice had a significantly larger stem cell population and exhibited higher and transplantable proliferative potential. In contrast, Jab1(+/-) mice, which express approximately 70% as much JAB1 protein as their wild-type littermates, showed inefficient hematopoiesis. Expression of the tumor suppressor p16(INK4a) was inversely correlated with that of JAB1, and the oncoprotein SMYD3, a newly identified JAB1 interactor, suppressed transcription of p16 in cooperation with JAB1. Thus, the expression and function of JAB1 are critical for the proliferation and maintenance of hematopoietic progenitors.

CSN5 isopeptidase activity links COP9 signalosome activation to breast cancer progression

CSN5 has been implicated as a candidate oncogene in human breast cancers by genetic linkage with activation of the poor-prognosis, wound response gene expression signature. CSN5 is a subunit of the eight-protein COP9 signalosome, a signaling complex with multiple biochemical activities; the mechanism of CSN5 action in cancer development remains poorly understood. Here, we show that CSN5 isopeptidase activity is essential for breast epithelial transformation and progression. Amplification of CSN5 is required for transformation of primary human breast epithelial cells by defined oncogenes. The transforming effects of CSN5 require CSN subunits for assembly of the full COP9 signalosome and the isopeptidase activity of CSN5, which potentiates the transcriptional activity of MYC. Transgenic inhibition of CSN5 isopeptidase activity blocks breast cancer progression evoked by MYC and RAS in vivo. These results highlight CSN5 isopeptidase activity in breast cancer progression, suggesting it as a therapeutic target in aggressive human breast cancers.

Live or let die: E2F1 and PI3K pathways intersect to make life or death decisions

In the current issue of Cancer Cell, Hallstrom et al. show that a subset of targets of the growth regulatory transcription factor E2F1 are repressed by a serum-induced PI3K activation, explaining how apoptosis can be suppressed while simultaneously engaging a proliferation program.

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.

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.

JAB1 accelerates mitochondrial apoptosis by interaction with proapoptotic BclGs

The Bcl-2 family of proteins is the key regulators of cell apoptosis at the mitochondria level. The BH3-only pro-apoptotic member BclGs was unique among the family due to its highly specific expression in human testis and has been demonstrated to induce apoptosis dependent on the BH3 domain. However, the molecular mechanism of BclGs-induced apoptosis remains unclear. Here we show that overexpression of BclGs could induce Bax expression upregulation and translocation to mitochondria, cytochrome c release and activation of caspase-3. Moreover, we identified JAB1 as a novel BclGs-specific binding protein through a yeast two-hybrid screening in a human testis cDNA library. BclGs interacts with JAB1 both in vitro and in vivo. N-terminal region of BclGs (aa 1-67) was required for the interaction. Importantly, JAB1 and BclGs co-expression synergistically induces apoptosis. JAB1 could compete with Bcl-XL/Bcl-2 to bind to BclGs; thus, promote the apoptosis. RNAi-mediated knock-down of JAB1 results in the reduced proapoptotic activity of BclGs. Taken together, our results provided the first evidence that JAB1 is involved in the regulation of mitochondrial apoptotic pathway through specific interaction with BclGs.

A highly conserved regulatory element controls hematopoietic expression of GATA-2 in zebrafish

Background

GATA-2 is a transcription factor required for hematopoietic stem cell survival as well as for neuronal development in vertebrates. It has been shown that specific expression of GATA-2 in blood progenitor cells requires distal cis-acting regulatory elements. Identification and characterization of these elements should help elucidating transcription regulatory mechanisms of GATA-2 expression in hematopoietic lineage.

Results

By pair-wise alignments of the zebrafish genomic sequences flanking GATA-2 to orthologous regions of fugu, mouse, rat and human genomes, we identified three highly conserved non-coding sequences in the genomic region flanking GATA-2, two upstream of GATA-2 and another downstream. Using both transposon and bacterial artificial chromosome mediated germline transgenic zebrafish analyses, one of the sequences was established as necessary and sufficient to direct hematopoietic GFP expression in a manner that recapitulates that of GATA-2. In addition, we demonstrated that this element has enhancer activity in mammalian myeloid leukemia cell lines, thus validating its functional conservation among vertebrate species. Further analysis of potential transcription factor binding sites suggested that integrity of the putative HOXA3 and LMO2 sites is required for regulating GATA-2/GFP hematopoietic expression.

Conclusion

Regulation of GATA-2 expression in hematopoietic cells is likely conserved among vertebrate animals. The integrated approach described here, drawing on embryological, transgenesis and computational methods, should be generally applicable to analyze tissue-specific gene regulation involving distal DNA cis-acting elements.

EGFR Signaling Inhibits E2F1-Induced Apoptosis in Vivo: Implications for Cancer Therapy

The retinoblastoma tumor suppressor (RB) restricts cell proliferation by regulating members of the E2F family of transcription factors. In human tumors RB is often inactivated, resulting in aberrant E2F-dependent transcription and uncontrolled proliferation. One of the E2F proteins, E2F1, can also induce apoptosis. The extent of E2F1-induced apoptosis is known to be tissue- and cell-specific, but until now, it has been unclear what variables determine cellular sensitivity to E2F1-induced apoptosis in vivo. A recent study reveals epidermal growth factor receptor (EGFR) signaling to be one such variable, as EGFR signaling cooperates with RB in inhibiting E2F1-induced apoptosis. This finding raises the possibility that therapeutic manipulation of EGFR signaling may specifically trigger the death of cancer cells with inactive RB, thereby enabling "targeted" cancer treatments.

Retinoblastoma protein regulation by the COP9 signalosome

Similar to their human counterparts, the Drosophila Rbf1 and Rbf2 Retinoblastoma family members control cell cycle and developmentally regulated gene expression. Increasing evidence suggests that Rbf proteins rely on multiprotein complexes to control target gene transcription. We show here that the developmentally regulated COP9 signalosome (CSN) physically interacts with Rbf2 during embryogenesis. Furthermore, the CSN4 subunit of the COP9 signalosome co-occupies Rbf target gene promoters with Rbf1 and Rbf2, suggesting an active role for the COP9 signalosome in transcriptional regulation. The targeted knockdown of individual CSN subunits leads to diminished Rbf1 and Rbf2 levels and to altered cell cycle progression. The proteasome-mediated destruction of Rbf1 and Rbf2 is increased in cells and embryos with diminished COP9 activity, suggesting that the COP9 signalosome protects Rbf proteins during embryogenesis. Previous evidence has linked gene activation to protein turnover via the promoter-associated proteasome. Our findings suggest that Rbf repression may similarly involve the proteasome and the promoter-associated COP9 signalosome, serving to extend Rbf protein lifespan and enable appropriate programs of retinoblastoma gene control during development.

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.

Molecular cloning and characterization of human Aph2 gene, involved in AP-1 regulation by interaction with JAB1

A human Aph2 gene (hAph2) was identified and cloned from a human placenta cDNA library. Bioinformatics analysis revealed hAPH2 protein shares 96% identity with mouse APH2 and contains a zf-DHHC domain (148-210aa), which is always involved in protein-protein or protein-DNA interaction. Differential expression patterns of hAph2 mRNA were observed in normal human tissues. Yeast two-hybrid screening found another hAPH2-interacting protein JAB1. The zf-DHHC domain of hAPH2 and the C-terminal of JAB1 were confirmed to be critical for the interaction. Fused with GFP and expressed in COS-7, NIH/3T3 and SMMC-7721 cell lines, hAPH2 showed predominant distribution in the cytoplasm and co-localized with JAB1 around the nucleus. Furthermore, overexpression of hAPH2 could increase apoptosis of COS-7 cells and negatively regulate JAB1-induced activation of AP-1 in a concentration dependent manner. The expression level of c-jun was also down-regulated by overexpression of hAPH2 in COS-7 cells. These data showed some basic characterization and function of hAph2 (hAPH2), dependent or independent with JAB1.