The ETS protein MEF is regulated by phosphorylation-dependent proteolysis via the protein-ubiquitin ligase SCFSkp2

SKping cell cycle regulation: role of ubiquitin ligase SKP2 in hematological malignancies

SKP2 (S-phase kinase-associated protein 2) is a member of the F-box family of substrate-recognition subunits in the SCF ubiquitin-protein ligase complexes. It is associated with ubiquitin-mediated degradation in the mammalian cell cycle components and other target proteins involved in cell cycle progression, signal transduction, and transcription. Being an oncogene in solid tumors and hematological malignancies, it is frequently associated with drug resistance and poor disease outcomes. In the current review, we discussed the novel role of SKP2 in different hematological malignancies. Further, we performed a limited in-silico analysis to establish the involvement of SKP2 in a few publicly available cancer datasets. Interestingly, our study identified Skp2 expression to be altered in a cancer-specific manner. While it was found to be overexpressed in several cancer types, few cancer showed a down-regulation in SKP2. Our review provides evidence for developing novel SKP2 inhibitors in hematological malignancies. We also investigated the effect of SKP2 status on survival and disease progression. In addition, the role of miRNA and its associated families in regulating Skp2 expression was explored. Subsequently, we predicted common miRNAs against Skp2 genes by using miRNA-predication tools. Finally, we discussed current approaches and future prospective approaches to target the Skp2 gene by using different drugs and miRNA-based therapeutics applications in translational research.

An ELF4 hypomorphic variant results in NK cell deficiency

NK cell deficiencies (NKD) are a type of primary immune deficiency in which the major immunologic abnormality affects NK cell number, maturity, or function. Since NK cells contribute to immune defense against virally infected cells, patients with NKD experience higher susceptibility to chronic, recurrent, and fatal viral infections. An individual with recurrent viral infections and mild hypogammaglobulinemia was identified to have an X-linked damaging variant in the transcription factor gene ELF4. The variant does not decrease expression but disrupts ELF4 protein interactions and DNA binding, reducing transcriptional activation of target genes and selectively impairing ELF4 function. Corroborating previous murine models of ELF4 deficiency (Elf4-/-) and using a knockdown human NK cell line, we determined that ELF4 is necessary for normal NK cell development, terminal maturation, and function. Through characterization of the NK cells of the proband, expression of the proband's variant in Elf4-/- mouse hematopoietic precursor cells, and a human in vitro NK cell maturation model, we established this ELF4 variant as a potentially novel cause of NKD.

ELF4 is critical to zygotic gene activation and epigenetic reprogramming during early embryonic development in pigs

Zygotic gene activation (ZGA) and epigenetic reprogramming are critical in early embryonic development in mammals, and transcription factors are involved in regulating these events. However, the effects of ELF4 on porcine embryonic development remain unclear. In this study, the expression of ELF4 was detected in early porcine embryos and different tissues. By knocking down ELF4, the changes of H3K9me3 modification, DNA methylation and ZGA-related genes were analyzed. Our results showed that ELF4 was expressed at all stages of early porcine embryos fertilized in vitro (IVF), with the highest expression level at the 8-cell stage. The embryonic developmental competency and blastocyst quality decreased after ELF4 knockdown (20.70% control vs. 17.49% si-scramble vs. 2.40% si-ELF4; p < 0.001). Knockdown of ELF4 induced DNA damage at the 4-cell stage. Interfering with ELF4 resulted in abnormal increases in H3K9me3 and DNA methylation levels at the 4-cell stage and inhibited the expression of genes related to ZGA. These results suggest that ELF4 affects ZGA and embryonic development competency in porcine embryos by maintaining genome integrity and regulating dynamic changes of H3K9me3 and DNA methylation, and correctly activating ZGA-related genes to promote epigenetic reprogramming. These results provide a theoretical basis for further studies on the regulatory mechanisms of ELF4 in porcine embryos.

Analysis of the FBXO7 promoter reveals overlapping Pax5 and c-Myb binding sites functioning in B cells

Fbxo7 is a key player in the differentiation and function of numerous blood cell types, and in neurons, oligodendrocytes and spermatocytes. In an effort to gain insight into the physiological and pathological settings where Fbxo7 is likely to play a key role, we sought to define the transcription factors which direct FBXO7 expression. Using sequence alignments across 28 species, we defined the human FBXO7 promoter and found that it contains two conserved regions enriched for multiple transcription factor binding sites. Many of these have roles in either neuronal or haematopoietic development. Using various FBXO7 promoter reporters, we found ELF4, Pax5 and c-Myb have functional binding sites that activate transcription. We find endogenous Pax5 is bound to the FBXO7 promoter in pre-B cells, and that the exogenous expression of Pax5 represses Fbxo7 transcription in early pro-B cells.

Ubiquitin-Mediated Control of ETS Transcription Factors: Roles in Cancer and Development

Genome expansion, whole genome and gene duplication events during metazoan evolution produced an extensive family of ETS genes whose members express transcription factors with a conserved winged helix-turn-helix DNA-binding domain. Unravelling their biological roles has proved challenging with functional redundancy manifest in overlapping expression patterns, a common consensus DNA-binding motif and responsiveness to mitogen-activated protein kinase signalling. Key determinants of the cellular repertoire of ETS proteins are their stability and turnover, controlled largely by the actions of selective E3 ubiquitin ligases and deubiquitinases. Here we discuss the known relationships between ETS proteins and enzymes that determine their ubiquitin status, their integration with other developmental signal transduction pathways and how suppression of ETS protein ubiquitination contributes to the malignant cell phenotype in multiple cancers.

IL6R is a target of miR-197 in human keratinocytes

Psoriasis is a chronic inflammatory disorder with cutaneous and systemic manifestations and substantial negative effects on patients' quality of life. MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression that play a role in the pathogenesis of psoriasis. Previously studies, from others and by us, highlighted specific miRNAs that are dysregulated in psoriatic lesions. MicroRNA-197-3p (miR-197) expression is downregulated in psoriatic lesions compared to normal or uninvolved skin in patients with psoriasis. We have previously reported that miR-197 could modulate IL-22 and IL-17 signalling in psoriasis. Herein, we identify additional biochemical targets of miR-197 in psoriasis. We applied a transcriptome-wide biochemical approach, Protein argonaute-2 photoactivatable ribonucleoside-enhanced crosslinking and immunoprecipitation (Ago2 PAR-CLIP), to search for new targets of miR-197 in live keratinocytes, and validated its results using reporter assay and analysing by Western blot protein levels in cells overexpressing miR-197. Ago2 PAR-CLIP identified biochemical targets of miR-197, including the alpha subunit of the IL-6 receptor (IL6R). This work provides evidence that IL6R in bona-fide biochemical target of miR-197. IL6R is known to be up-regulated in psoriasis and even was considered as a possible therapeutic target. From the present data and our previous studies, it appears that miR-197 is a major regulator of the interaction between immune system cells and keratinocytes.

The Skp2 Pathway: A Critical Target for Cancer Therapy

Strictly regulated protein degradation by ubiquitin-proteasome system (UPS) is essential for various cellular processes whose dysregulation is linked to serious diseases including cancer. Skp2, a well characterized component of Skp2-SCF E3 ligase complex, is able to conjugate both K48-linked ubiquitin chains and K63-linked ubiquitin chains on its diverse substrates, inducing proteasome mediated proteolysis or modulating the function of tagged substrates respectively. Overexpression of Skp2 is observed in various human cancers associated with poor survival and adverse therapeutic outcomes, which in turn suggests that Skp2 engages in tumorigenic activity. To that end, the oncogenic properties of Skp2 are demonstrated by various genetic mouse models, highlighting the potential of Skp2 as a target for tackling cancer. In this article, we will describe the downstream substrates of Skp2 as well as upstream regulators for Skp2-SCF complex activity. We will further summarize the comprehensive oncogenic functions of Skp2 while describing diverse strategies and therapeutic platforms currently available for developing Skp2 inhibitors.

Regulation of Stem Cells by Cullin-RING Ligase

Stem cells can remain quiescent, self-renewal, and differentiate into many types of cells and even cancer stem cells. The coordination of these complex processes maintains the homeostasis of the organism. Ubiquitination is an important posttranslational modification process that regulates protein stability and activity. The ubiquitination levels of stem cell-associated proteins are closely related with stem cell characteristics. Cullin-RING Ligases (CRLs) are the largest family of E3 ubiquitin ligases, accounting for approximately 20% of proteins degraded by proteasome. In this review, we discuss the role of CRLs in stem cell homeostasis, self-renewal, and differentiation and expound their ubiquitination substrates. In addition, we also discuss the effect of CRLs on the formation of cancer stem cells that may provide promising therapy strategies for cancer.

Roles and regulations of the ETS transcription factor ELF4/MEF

Most E26 transformation-specific (ETS) transcription factors are involved in the pathogenesis and progression of cancer. This is in part due to the roles of ETS transcription factors in basic biological processes such as growth, proliferation, and differentiation, and also because of their regulatory functions that have physiological relevance in tumorigenesis, immunity, and basal cellular homoeostasis. A member of the E74-like factor (ELF) subfamily of the ETS transcription factor family—myeloid elf-1-like factor (MEF), designated as ELF4—has been shown to be critically involved in immune response and signalling, osteogenesis, adipogenesis, cancer, and stem cell quiescence. ELF4 carries out these functions as a transcriptional activator or through interactions with its partner proteins. Mutations in ELF4 cause aberrant interactions and induce downstream processes that may lead to diseased cells. Knowing how ELF4 impinges on certain cellular processes and how it is regulated in the cells can lead to a better understanding of the physiological and pathological consequences of modulated ELF4 activity.

The transcription factor MEF/Elf4 is dually modulated by p53-MDM2 axis and MEF-MDM2 autoregulatory mechanism

Myeloid Elf-1-like factor (MEF) or Elf4 is an ETS transcription factor that activates innate immunity-associated genes such as lysozyme (LYZ), human β-defensin 2 (HβD2), and interleukin-8 (IL-8) in epithelial cells and is also known to influence cell cycle progression. MEF is transcriptionally activated by E2F1, but the E2F1-mediated transcriptional activation is inhibited by p53 through E2F1-p53 protein interaction. Although the transcriptional activation of MEF has been investigated in depth, its post-translational regulation is not well explored. By overexpressing MEF cDNA in human cell lines, here we show that MEF protein expression is suppressed by p53. By screening a number of E3 ligases regulated by p53, we found that MDM2 is involved in the effect of p53 on MEF. MDM2 is transcriptionally activated by p53 and interacts with MEF protein to enhance MEF degradation. MDM2 reduces MEF protein expression, as well as stability and function of MEF as transcriptional activator. Furthermore, MDM2 was able to down-regulate MEF in the absence of p53, indicating a p53-independent effect on MEF. Notably, MEF transcriptionally activates MDM2, which was previously demonstrated to be the mechanism by which MEF suppresses the p53 protein. These results reveal that in addition to the potential of MEF to down-regulate p53 by transcriptionally activating E3 ligase MDM2, MEF participates with MDM2 in a novel autoregulatory feedback loop to regulate itself. Taken together with the findings on the effect of p53 on MEF, these data provide evidence that the p53-MDM2-MEF axis is a feedback mechanism that exquisitely controls the balance of these transcriptional regulators.

Targeting the oncogenic E3 ligase Skp2 in prostate and breast cancer cells with a novel energy restriction-mimetic agent

Substantial evidence supports the oncogenic role of the E3 ubiquitin ligase S-phase kinase-associated protein 2 (Skp2) in many types of cancers through its ability to target a broad range of signaling effectors for ubiquitination. Thus, this oncogenic E3 ligase represents an important target for cancer drug discovery. In this study, we report a novel mechanism by which CG-12, a novel energy restriction-mimetic agent (ERMA), down-regulates the expression of Skp2 in prostate cancer cells. Pursuant to our previous finding that upregulation of β-transducin repeat-containing protein (β-TrCP) expression represents a cellular response in cancer cells to ERMAs, including CG-12 and 2-deoxyglucose, we demonstrated that this β-TrCP accumulation resulted from decreased Skp2 expression. Evidence indicates that Skp2 targets β-TrCP for degradation via the cyclin-dependent kinase 2-facilitated recognition of the proline-directed phosphorylation motif ⁴¹²SP. This Skp2 downregulation was attributable to Sirt1-dependent suppression of COP9 signalosome (Csn)5 expression in response to CG-12, leading to increased cullin 1 neddylation in the Skp1-cullin1-F-box protein complex and consequent Skp2 destabilization. Moreover, we determined that Skp2 and β-TrCP are mutually regulated, providing a feedback mechanism that amplifies the suppressive effect of ERMAs on Skp2. Specifically, cellular accumulation of β-TrCP reduced the expression of Sp1, a β-TrCP substrate, which, in turn, reduced Skp2 gene expression. This Skp2-β-TrCP-Sp1 feedback loop represents a novel crosstalk mechanism between these two important F-box proteins in cancer cells with aberrant Skp2 expression under energy restriction, which provides a proof-of-concept that the oncogenic Csn5/Skp2 signaling axis represents a “druggable” target for this novel ERMA.

Marking the tempo for myogenesis: Pax7 and the regulation of muscle stem cell fate decisions

Post-natal growth and regeneration of skeletal muscle is highly dependent on a population of resident myogenic precursors known as satellite cells. Transcription factors from the Pax gene family, Pax3 and Pax7, are critical for satellite cell biogenesis, survival and potentially self-renewal; however, the underlying molecular mechanisms remain unsolved. This is particularly true in the case of Pax7, which appears to regulate myogenesis at multiple levels. Accordingly, recent data have highlighted the importance of a functional relationship between Pax7 and the MyoD family of muscle regulatory transcription factors during normal muscle formation and disease. Here we will critically review key findings suggesting that Pax7 may play a dual role by promoting resident muscle progenitors to commit to the skeletal muscle lineage while preventing terminal differentiation, thus keeping muscle progenitors poised to differentiate upon environmental cues. In addition, potential regulatory mechanisms for the control of Pax7 activity will be proposed.

Novel roles of Skp2 E3 ligase in cellular senescence, cancer progression, and metastasis

S-phase kinase-associated protein 2 (Skp2) belongs to the F-box protein family. It is a component of the SCF E3 ubiquitin ligase complex. Skp2 has been shown to regulate cellular proliferation by targeting several cell cycle-regulated proteins for ubiquitination and degradation, including cyclin-dependent kinase inhibitor p27. Skp2 has also been demonstrated to display an oncogenic function since its overexpression has been observed in many human cancers. This review discusses the recent discoveries on the novel roles of Skp2 in regulating cellular senescence, cancer progression, and metastasis, as well as the therapeutic potential of targeting Skp2 for human cancer treatment.

Cullins and cancer

The cullin family of ubiquitin ligases can potentially assemble hundreds of RING-type E3 complexes (CRLs) by utilizing different substrate receptors that share common interaction domains. Cullin receptors dictate substrate specificity, and cullin-mediated substrate degradation controls a wide range of cellular processes, including proliferation, differentiation, and apoptosis. Dysregulation of cullin activity has been shown to contribute to oncogenesis through the accumulation of oncoproteins or the excessive degradation of tumor suppressors. In this review, we will discuss cullin complexes and their substrates, the regulatory pathways that affect cullin activity, and the mechanisms by which cullins may facilitate or inhibit carcinogenesis.

The mef/elf4 transcription factor fine tunes the DNA damage response

The ATM kinase plays a critical role in initiating the DNA damage response that is triggered by genotoxic stresses capable of inducing DNA double-strand breaks. Here, we show that ELF4/MEF, a member of the ETS family of transcription factors, contributes to the persistence of γH2AX DNA damage foci and promotes the DNA damage response leading to the induction of apoptosis. Conversely, the absence of ELF4 promotes the faster repair of damaged DNA and more rapid disappearance of γH2AX foci in response to γ-irradiation, leading to a radio-resistant phenotype despite normal ATM phosphorylation. Following γ-irradiation, ATM phosphorylates ELF4, leading to its degradation; a mutant form of ELF4 that cannot be phosphorylated by ATM persists following γ-irradiation, delaying the resolution of γH2AX foci and triggering an excessive DNA damage response. Thus, although ELF4 promotes the phosphorylation of H2AX by ATM, its activity must be dampened by ATM-dependent phosphorylation and degradation to avoid an excessive DNA damage response.

MicroRNAs control intestinal epithelial differentiation, architecture, and barrier function

BACKGROUND & AIMS

Whereas the importance of microRNA (miRNA) for the development of several tissues is well established, its role in the intestine is unknown. We aimed to quantify the complete miRNA expression profile of the mammalian intestinal mucosa and to determine the contribution of miRNAs to intestinal homeostasis using genetic means.

METHODS

We determined the miRNA transcriptome of the mouse intestinal mucosa using ultrahigh throughput sequencing. Using high-throughput sequencing of RNA isolated by cross-linking immunoprecipitation (HITS-CLIP), we identified miRNA-messenger RNA target relationships in the jejunum. We employed gene ablation of the obligatory miRNA-processing enzyme Dicer1 to derive mice deficient for all miRNAs in intestinal epithelia.

RESULTS

miRNA abundance varies dramatically in the intestinal mucosa, from 1 read per million to 250,000. Of the 453 miRNA families identified, mmu-miR-192 is the most highly expressed in both the small and large intestinal mucosa, and there is a 53% overlap in the top 15 expressed miRNAs between the 2 tissues. The intestinal epithelium of Dicer1(loxP/loxP);Villin-Cre mutant mice is disorganized, with a decrease in goblet cells, a dramatic increase in apoptosis in crypts of both jejunum and colon, and accelerated jejunal cell migration. Furthermore, intestinal barrier function is impaired in Dicer1-deficient mice, resulting in intestinal inflammation with lymphocyte and neutrophil infiltration. Our list of miRNA-messenger RNA targeting relationships in the small intestinal mucosa provides insight into the molecular mechanisms behind the phenotype of Dicer1 mutant mice.

CONCLUSIONS

We have identified all intestinal miRNAs and shown using gene ablation of Dicer1 that miRNAs play a vital role in the differentiation and function of the intestinal epithelium.

MEF/ELF4 transactivation by E2F1 is inhibited by p53

Myeloid elf-1-like factor (MEF) or Elf4 is an E-twenty-six (ETS)-related transcription factor with strong transcriptional activity that influences cellular senescence by affecting tumor suppressor p53. MEF downregulates p53 expression and inhibits p53-mediated cellular senescence by transcriptionally activating MDM2. However, whether p53 reciprocally opposes MEF remains unex-plored. Here, we show that MEF is modulated by p53 in human cells and mice tissues. MEF expression and promoter activity were suppressed by p53. While we found that MEF promoter does not contain p53 response elements, intriguingly, it contains E2F consensus sites. Subsequently, we determined that E2F1 specifically binds to MEF promoter and transactivates MEF. Nevertheless, E2F1 DNA binding and transactivation of MEF promoter was inhibited by p53 through the association between p53 and E2F1. Furthermore, we showed that activation of p53 in doxorubicin-induced senescent cells increased E2F1 and p53 interaction, diminished E2F1 recruitment to MEF promoter and reduced MEF expression. These observations suggest that p53 downregulates MEF by associating with and inhibiting the binding activity of E2F1, a novel transcriptional activator of MEF. Together with previous findings, our present results indicate that a negative regulatory mechanism exists between p53 and MEF.

Deciphering the transcriptional complex critical for RhoA gene expression and cancer metastasis

The RhoA GTPase is crucial in numerous biological functions and is linked to cancer metastasis. However, the understanding of the molecular mechanism responsible for RhoA transcription is still very limited. Here we show that RhoA transcription is orchestrated by the Myc-Skp2-Miz1-p300 transcriptional complex. Skp2 cooperates with Myc to induce RhoA transcription by recruiting Miz1 and p300 to the RhoA promoter independently of Skp1-Cullin-F-box protein containing complex (SCF)-Skp2 E3 ligase activity. Deficiency of this complex results in impairment in RhoA expression, cell migration, invasion, and breast cancer metastasis, recapitulating the phenotypes observed in RhoA knockdown, and RhoA restoration rescues the defect in cell invasion. Overexpression of the Myc-Skp2-Miz1 complex is found in metastatic human cancers and is correlated with RhoA expression. Our study provides insight into how oncogenic Skp2 and Myc coordinate to induce RhoA transcription and establishes a novel SCF-Skp2 E3-ligase-independent function for oncogenic Skp2 in transcription and cancer metastasis.

Identification of two critically deleted regions within chromosome segment 7q35-q36 in EVI1 deregulated myeloid leukemia cell lines

Chromosomal rearrangements involving the EVI1 proto-oncogene are a recurrent finding in myeloid leukemias and are indicative of a poor prognosis. Rearrangements of the EVI1 locus are often associated with monosomy 7 or cytogenetic detectable deletions of part of 7q. As EVI1 overexpression alone is not sufficient to induce leukemia, loss of a 7q tumour suppressor gene might be a required cooperating event. To test this hypothesis, we performed high-resolution array comparative genomic hybridization analysis of twelve EVI1 overexpressing patients and three EVI1 deregulated cell lines to search for 7q submicroscopic deletions. This analysis lead to the delineation of two critical regions, one of 0.39 Mb on 7q35 containing the CNTNAP2 gene and one of 1.33 Mb on chromosome bands 7q35-q36 comprising nine genes in EVI1 deregulated cell lines. These findings open the way to further studies aimed at identifying the culprit EVI1 implicated tumour suppressor genes on 7q.

The E3 ubiquitin ligase complex component COP1 regulates PEA3 group member stability and transcriptional activity

In this study, we report that the PEA3 group members interact with the mammalian really interesting new gene (RING) E3 ubiquitin ligase constitutive photomorphogenetic 1 (COP1), which mediates ubiquitylation and subsequent proteasome degradation of the p53 and c-Jun transcription factors. This interaction is mediated by the central region of COP1 including the coiled-coil domain and two COP1-interacting consensus motifs localized in the well-conserved N-terminal transactivation domain of the PEA3 group members. At the transcriptional level, COP1 reduces the transcriptional activity of ERM and the two other PEA3 group proteins on Ets-responsive reporter genes; this effect being dependent on the RING domain of COP1 and the two COP1-interacting motifs of ERM. Reduced transcriptional activity was, however, not related to COP1-induced changes in ERM stability. In fact, increased ubiquitylation and subsequent proteasome-mediated degradation of ERM is achieved only when COP1 is expressed with DET1, a key COP1 partner within the ubiquitylation complex. Conversely, we show that the depletion of COP1 or DET1 by small interference RNA (siRNA) in U2OS cells stabilizes endogenous ERM whereas only COP1 knockdown enhances expression of ICAM-1, a gene regulated by this transcription factor. These results indicate that COP1 is a complex regulator of ERM and the two other PEA3 group members.

A review of post-translational modifications and subcellular localization of Ets transcription factors: possible connection with cancer and involvement in the hypoxic response

Post-translational modifications and subcellular localizations modulate transcription factors, generating a code that is deciphered into an activity. We describe our current understanding of these processes for Ets factors, which have recently been recognized for their importance in various biological processes. We present the global picture for the family, and then focus on particular aspects related to cancer and hypoxia. The analysis of Post-translational modification and cellular localization is only beginning to enter the age of "omic," high content, systems biology. Our snap-shots of particularly active fields point to the directions in which new techniques will be needed, in our search for a more complete description of regulatory pathways.

Association of luteinizing hormone receptor gene expression with cell cycle progression in granulosa cells

During hormonally induced ovarian follicle growth, granulosa cell proliferation increases and returns to baseline prior to the administration of an ovulatory stimulus. Several key genes appear to follow a similar pattern, including the luteinizing hormone receptor (LHCGR), suggesting an association between cell cycle progression and gene expression. The expression of LHCGR mRNA in granulosa cells isolated from immature rats and treated in culture with FSH increased in a time-dependent manner, whereas administration of the cell cycle inhibitor mimosine completely suppressed expression. Although forskolin was able to induce luteinization in cells treated with mimosine, human chorionic gonadotropin had no effect, indicating the functional loss of LHCGR. The effects of mimosine on cell cycle progression and LHCGR mRNA expression were reversible within 24 h of mimosine removal. Cell cycle inhibition did not alter the stability of LHCGR mRNA, indicating that the primary effect was at the transcriptional level. To determine whether the relationship between LHCGR expression and cell cycle were relevant in vivo, immature rats were given a bolus of PMSG, followed by a second injection of either saline or PMSG 24 h later to augment levels of proliferation. The expression of LHCGR mRNA was elevated in the ovaries of animals receiving a supplement of PMSG. Mimosine also blocked cell cycle progression and LHCGR mRNA expression in macaque granulosa cells isolated following controlled ovarian stimulation cycles and in two different mouse Leydig tumor lines. These data collectively indicate that LHCGR mRNA is expressed as a function of the passage of cells across the G1-S phase boundary.

Cancer-related transcriptional targets of the circadian gene NPAS2 identified by genome-wide ChIP-on-chip analysis

The transcription factor NPAS2 is one of nine human core circadian genes that influence a variety of biological processes by regulating the 24-h circadian rhythm. Recently, it has been shown that NPAS2 is a risk biomarker in human cancers and plays a role in tumorigenesis by affecting cancer-related gene expression, and relevant biological pathways. However, it is difficult to study the biological involvement of NPAS2 in cancer development, as little is known about its direct transcriptional targets. The aim of the current study is to create a transcriptional profile of genes regulated by NPAS2, using a human binding ChIP-on-chip analysis of NPAS2 in MCF-7 cells. This genome-wide mapping approach identified 26 genes that contain potential NPAS2 binding regions. Subsequent real-time PCR assays confirmed 16 of these targets, and 9 of these genes (ARHGAP29, CDC25A, CDKN2AIP, CX3CL1, ELF4, GNAL, KDELR1, POU4F2, and THRA) have a known role in tumorigenesis. In addition, a networking analysis of these validated NPAS2 targets revealed that all nine genes, together with REN, are involved in a "Cancer, Cell cycle, Neurological Disease" network. These results report the first list of direct transcriptional targets of NPAS2 and will shed light on the role of circadian genes in tumorigenesis.

Transcription factor ELF4 controls the proliferation and homing of CD8+ T cells via the Krüppel-like factors KLF4 and KLF2

Transcription factors that regulate the quiescence, proliferation and homing of lymphocytes are critical for effective immune system function. Here we demonstrate that the transcription factor ELF4 directly activated the tumor suppressor KLF4 'downstream' of T cell antigen receptor signaling to induce cell cycle arrest in naive CD8(+) T cells. Elf4- and Klf4-deficient mice accumulated CD8(+)CD44(hi) T cells during steady-state conditions and generated more memory T cells after immunization. The homeostatic population expansion of CD8(+)CD44(hi) T cells in Elf4-null mice resulted in a redistribution of cells to nonlymphoid tissue because of lower expression of the transcription factor KLF2 and the surface proteins CCR7 and CD62L. Our work describes the combinatorial effect of lymphocyte-intrinsic factors on the homeostasis, activation and homing of T cells.

ELF4/MEF activates MDM2 expression and blocks oncogene-induced p16 activation to promote transformation

Several ETS transcription factors, including ELF4/MEF, can function as oncogenes in murine cancer models and are overexpressed in human cancer. We found that Elf4/Mef activates Mdm2 expression; thus, lack of or knockdown of Elf4/Mef reduces Mdm2 levels in mouse embryonic fibroblasts (mef's), leading to enhanced p53 protein accumulation and p53-dependent senescence. Even though p53 is absent in Elf4(-/-) p53(-/-) mef's, neither oncogenic H-Ras(V12) nor c-myc can induce transformation of these cells. This appears to relate to the INK4a/ARF locus; both p19(ARF) and p16 are increased in Elf4(-/-) p53(-/-) mef's, and expression of Bmi-1 or knockdown of p16 in this context restores H-Ras(V12)-induced transformation. Thus, ELF4/MEF promotes tumorigenesis by inhibiting both the p53 and p16/Rb pathways.

Deregulated proteolysis by the F-box proteins SKP2 and beta-TrCP: tipping the scales of cancer

The maintenance and preservation of distinct phases during the cell cycle is a highly complex and coordinated process. It is regulated by phosphorylation--through the activity of cyclin-dependent kinases (CDKs)--and protein degradation, which occurs through ubiquitin ligases such as SCF (SKP1-CUL1-F-box protein) complexes and APC/C (anaphase-promoting complex/cyclosome). Here, we explore the functionality and biology of the F-box proteins, SKP2 (S-phase kinase-associated protein 2) and beta-TrCP (beta-transducin repeat-containing protein), which are emerging as important players in cancer biogenesis owing to the deregulated proteolysis of their substrates.

Cul4A is required for hematopoietic stem-cell engraftment and self-renewal

Several hematopoietic stem-cell (HSC) regulators are controlled by ubiquitin-mediated proteolysis, so the ubiquitin pathway might modulate HSC function. However, this hypothesis has not been formally tested. Cul4A encodes a core subunit of one ubiquitin ligase. Whereas Cul4A-deficient embryos die in utero, Cul4A-haploinsufficient mice are viable but exhibit abnormal hematopoiesis (fewer erythroid and primitive myeloid progenitors). Given these data, we examined whether Cul4A(+/-) HSCs might also be impaired. Using bone marrow transplantation assays, we determined that Cul4A(+/-) HSCs exhibit defects in engraftment and self-renewal capacity. These studies are the first to demonstrate that ubiquitin-mediated protein degradation is important for HSC function. Further, they indicate that a Cul4A ubiquitin ligase targets for degradation one or multiple HSC regulators.

Cell-cycle-dependent oscillation of GATA2 expression in hematopoietic cells

In vitro manipulation of hematopoietic stem cells (HSCs) is a key issue in both transplantation therapy and regenerative medicine, and thus new methods are required to achieve HSC expansion with self-renewal. GATA2 is a transcription factor controlling pool size of HSCs. Of interest, continuous overexpression of GATA2 does not induce HSC proliferation. In this report, we demonstrate that GATA2 expression, in leukemic and normal hematopoietic cells, oscillates during the cell cycle, such that expression is high in S phase but low in G(1)/S and M phase. GATA2 binding to target Bcl-X gene also oscillates in accordance with GATA2 expression. Using a green fluorescent protein (GFP)-GATA2 fusion protein, we demonstrate cell-cycle-specific activity of proteasome-dependent degradation of GATA2. Immunoprecipitation/immunoblotting analysis demonstrated phosphorylation of GATA2 at cyclin-dependent kinase (Cdk)-consensus motifs, S/T(0)P(+1), and interaction of GATA2 with Cdk2/cyclin A2-, Cdk2/cyclin A2-, and Cdk4/cyclin D1-phosphorylated GATA2 in vitro. Mutants in phosphorylation motifs exhibited altered expression profiles of GFP-GATA2 domain fusion proteins. These results indicate that GATA2 phosphorylation by Cdk/cyclin systems is responsible for the cell-cycle-dependent regulation of GATA2 expression, and suggest the possibility that a cell-cycle-specific "on-off" response of GATA2 expression may control hematopoietic-cell proliferation and survival.

Tumor promoting properties of the ETS protein MEF in ovarian cancer

We have previously shown that MEF (myeloid ELF1-like factor, also known as ELF4) functions as a transcriptional activator of the interleukin (IL)-8, perforin, granulocyte macrophage-colony stimulating factor (GM-CSF) and IL-3 genes in hematopoietic cells. MEF is also expressed in non-hematopoietic tissues including certain ovarian cancer cells. To define the function of MEF in these cells, we examined primary human ovarian epithelial tumors and found that MEF is expressed in a significant proportion of ovarian carcinomas, and in the CAOV3 and SKOV3 ovarian cancer cell lines, but not in normal ovarian surface epithelium. Manipulating MEF levels in these cell lines altered their behavior; reducing MEF levels, using short hairpin RNA expressing vectors, significantly inhibited the proliferation of SKOV3 and CAOV3 cells in culture, and impaired the anchorage-independent growth of CAOV3 cells. Overexpression of MEF in SKOV3 cells (via retroviral transduction) significantly increased their growth rate, enhanced colony formation in soft agar and promoted tumor formation in nude mice. The oncogenic activity of MEF was further shown by the ability of MEF to transform NIH3T3 cells, and induce their tumor formation in nude mice. MEF is an important regulator of the tumorigenic properties of ovarian cancer cells and could be used a therapeutic target in ovarian cancer.