Nucleolin is a protein kinase C-zeta substrate. Connection between cell surface signaling and nucleus in PC12 cells

Japanese encephalitis virus NS5 protein interacts with nucleolin to enhance the virus replication

Japanese encephalitis virus (JEV) is an arthropod-borne, plus-strand flavivirus causing viral encephalitis in humans with a high case fatality rate. The JEV non-structural protein 5 (NS5) with the RNA-dependent RNA polymerase activity interacts with the viral and host proteins to constitute the replication complex. We have identified the multifunctional protein Nucleolin (NCL) as one of the several NS5-interacting host proteins. We demonstrate the interaction and colocalization of JEV NS5 with NCL in the virus-infected HeLa cells. The siRNA-mediated knockdown of NCL indicated that it was required for efficient viral replication. Importantly, JEV grew to higher titers in cells over-expressing exogenous NCL, demonstrating its pro-viral role. We demonstrated that NS5 interacted with the RRM and GAR domains of NCL. We show that the NCL-binding aptamer AS1411 containing the G-quadruplex (GQ) structure and the GQ ligand BRACO-19 caused significant inhibition of JEV replication. The antiviral effect of AS1411 and BRACO-19 could be overcome in HeLa cells by the overexpression of exogenous NCL. We demonstrated that the synthetic RNAs derived from the 3'-NCR of JEV genomic RNA containing the GQ sequence could bind NCL in vitro. The replication complex binding to the 3'-NCR is required for the viral RNA synthesis. It is likely that NCL present in the replication complex destabilizes the GQ structures in the genomic RNA, thus facilitating the movement of the replication complex resulting in efficient virus replication.IMPORTANCEJapanese encephalitis virus (JEV) is endemic in most parts of South-East Asia and the Western Pacific region, causing epidemics of encephalitis with a high case fatality rate. While a tissue culture-derived JEV vaccine is available, no antiviral therapy exists. The JEV NS5 protein has RNA-dependent RNA polymerase activity. Together with several host and viral proteins, it constitutes the replication complex necessary for virus replication. Understanding the interaction of NS5 with the host proteins could help design novel antivirals. We identified Nucleolin (NCL) as a crucial host protein interactor of JEV NS5 having a pro-viral role in virus replication. The NS5-interacting NCL binds to the G-quadruplex (GQ) structure sequence in the 3'-NCR of JEV RNA. This may smoothen the movement of the replication complex along the genomic RNA, thereby facilitating the virus replication. This study is the first report on how NCL, a host protein, helps in JEV replication through GQ-binding.

AS1411-functionalized delivery nanosystems for targeted cancer therapy

Nucleolin (NCL) is a multifunctional nucleolar phosphoprotein harboring critical roles in cells such as cell proliferation, survival, and growth. The dysregulation and overexpression of NCL are related to various pathologic and oncological indications. These characteristics of NCL make it an ideal target for the treatment of various cancers. AS1411 is a synthetic quadruplex-forming nuclease-resistant DNA oligonucleotide aptamer which shows a considerably high affinity for NCL, therefore, being capable of inducing growth inhibition in a variety of tumor cells. The high affinity and specificity of AS1411 towards NCL make it a suitable targeting tool, which can be used for the functionalization of therapeutic payloaddelivery nanosystems to selectively target tumor cells. This review explores the advances in NCL-targeting cancer therapy through AS1411-functionalized delivery nanosystems for the selective delivery of a broad spectrum of therapeutic agents.

Nucleolin Overexpression Confers Increased Sensitivity to the Anti-Nucleolin Aptamer, AS1411

AS1411 is an antiproliferative DNA aptamer, which binds the ubiquitous protein, nucleolin. In this study, we show that constitutive overexpression of nucleolin confers increased sensitivity to the growth inhibitory effects of AS1411. HeLa cells overexpressing nucleolin have an increased growth rate and invasiveness relative to control cells. Nucleolin overexpressing cells demonstrate increased growth inhibition in response to the AS1411 treatment, which correlates with increased apoptosis and cell cycle arrest, when compared to non-transfected cells. AS1411 induces nucleolin expression at the RNA and protein level in HeLa cells, suggesting a feedback loop with important implications for the clinical use of AS1411.

G-quadruplex oligonucleotide AS1411 as a cancer-targeting agent: Uses and mechanisms

BACKGROUND

AS1411 is a 26-mer G-rich DNA oligonucleotide that forms a variety of G-quadruplex structures. It was identified based on its cancer-selective antiproliferative activity and subsequently determined to be an aptamer to nucleolin, a multifunctional protein that preferentially binds quadruplex nucleic acids and which is present at high levels on the surface of cancer cells. AS1411 has exceptionally efficient cellular internalization compared to non-quadruplex DNA sequences.

SCOPE OF REVIEW

Recent developments related to AS1411 will be examined, with a focus on its use for targeted delivery of therapeutic and imaging agents.

MAJOR CONCLUSIONS

Numerous research groups have used AS1411 as a targeting agent to deliver nanoparticles, oligonucleotides, and small molecules into cancer cells. Studies in animal models have demonstrated that AS1411-linked materials can accumulate selectively in tumors following systemic administration. The mechanism underlying the cancer-targeting ability of AS1411 is not completely understood, but recent studies suggest a model that involves: (1) initial uptake by macropinocytosis, a form of endocytosis prevalent in cancer cells; (2) stimulation of macropinocytosis by a nucleolin-dependent mechanism resulting in further uptake; and (3) disruption of nucleolin-mediated trafficking and efflux leading to cargoes becoming trapped inside cancer cells.

SIGNIFICANCE

Human trials have indicated that AS1411 is safe and can induce durable remissions in a few patients, but new strategies are needed to maximize its clinical impact. A better understanding of the mechanisms by which AS1411 targets and kills cancer cells may hasten the development of promising technologies using AS1411-linked nanoparticles or conjugates for cancer-targeted therapy and imaging. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.

LNA aptamer based multi-modal, Fe3O4-saturated lactoferrin (Fe3O4-bLf) nanocarriers for triple positive (EpCAM, CD133, CD44) colon tumor targeting and NIR, MRI and CT imaging

This is the first ever attempt to combine anti-cancer therapeutic effects of emerging anticancer biodrug bovine lactoferrin (bLf), and multimodal imaging efficacy of Fe3O4 nanoparticles (NPs) together, as a saturated Fe3O4-bLf. For cancer stem cell specific uptake of nanocapsules/nanocarriers (NCs), Fe3O4-bLf was encapsulated in alginate enclosed chitosan coated calcium phosphate (AEC-CP) NCs targeted (Tar) with locked nucleic acid (LNA) modified aptamers against epithelial cell adhesion molecule (EpCAM) and nucleolin markers. The nanoformulation was fed orally to mice injected with triple positive (EpCAM, CD133, CD44) sorted colon cancer stem cells in the xenograft cancer stem cell mice model. The complete regression of tumor was observed in 70% of mice fed on non-targeted (NT) NCs, with 30% mice showing tumor recurrence after 30 days, while only 10% mice fed with Tar NCs showed tumor recurrence indicating a significantly higher survival rate. From tumor tissue analyses of 35 apoptotic markers, 55 angiogenesis markers, 40 cytokines, 15 stem cell markers and gene expression studies of important signaling molecules, it was revealed that the anti-cancer mechanism of Fe3O4-bLf was intervened through TRAIL, Fas, Fas-associated protein with death domain (FADD) mediated phosphorylation of p53, to induce activation of second mitochondria-derived activator of caspases (SMAC)/DIABLO (inhibiting survivin) and mitochondrial depolarization leading to release of cytochrome C. Induction of apoptosis was observed by inhibition of the Akt pathway and activation of cytokines released from monocytes/macrophages and dendritic cells (interleukin (IL) 27, keratinocyte chemoattractant (KC)). On the other hand, the recurrence of tumor in AEC-CP-Fe3O4-bLf NCs fed mice mainly occurred due to activation of alternative pathways such as mitogen-activated protein kinases (MAPK)/extracellular signal-regulated kinases (ERK) and Wnt signaling leading to an increase in expression of survivin, survivin splice variant (survivin 2B) and other anti-apoptotic proteins Bad, Bcl-2 and XIAP. Apart from the promising anti-cancer efficacy and the exceptional tumor targeting ability observed by multimodal imaging using near-infrared (NIR) imaging, magnetic resonance imaging (MRI) and computerized tomographic (CT) techniques, these NCs also maintained the immunomodulatory benefits of bLf as they were able to increase the RBC, hemoglobin, iron calcium and zinc levels in mice.

Locked nucleic acid modified bi-specific aptamer-targeted nanoparticles carrying survivin antagonist towards effective colon cancer therapy

EpCAM and nucleolin translocate into the cytoplasm and nucleus that facilitates enhanced uptake of nanocarrier to specifically target cancer cells.

Aptamers as drug delivery vehicles

The benefits of directed and selective therapy for systemic treatment are reasons for increased interest in exploiting aptamers for cell-specific drug delivery. Nucleic acid based pharmaceuticals represent an interesting and novel tool to counter human diseases. Combining inhibitory potential and cargo transfer upon internalization, nanocarriers as well as various therapeutics including siRNAs, chemotherapeutics, photosensitizers, or proteins can be imported via these synthetic nucleic acids. However, widespread clinical application is still hampered by obstacles that must be overcome. In this review, we give an overview of applications and recent advances in aptamer-mediated drug delivery. We also introduce prominent selection methods as well as useful approaches in choice of drug and conjugation method. We discuss the challenges that need to be considered and present strategies that have been applied to achieve intracellular delivery of effectors transported by readily internalized aptamers.

Clostridium perfringens phospholipase C induced ROS production and cytotoxicity require PKC, MEK1 and NFκB activation

Clostridium perfringens phospholipase C (CpPLC), also called α-toxin, is the most toxic extracellular enzyme produced by this bacteria and is essential for virulence in gas gangrene. At lytic concentrations, CpPLC causes membrane disruption, whereas at sublytic concentrations this toxin causes oxidative stress and activates the MEK/ERK pathway, which contributes to its cytotoxic and myotoxic effects. In the present work, the role of PKC, ERK 1/2 and NFκB signalling pathways in ROS generation induced by CpPLC and their contribution to CpPLC-induced cytotoxicity was evaluated. The results demonstrate that CpPLC induces ROS production through PKC, MEK/ERK and NFκB pathways, the latter being activated by the MEK/ERK signalling cascade. Inhibition of either of these signalling pathways prevents CpPLC's cytotoxic effect. In addition, it was demonstrated that NFκB inhibition leads to a significant reduction in the myotoxicity induced by intramuscular injection of CpPLC in mice. Understanding the role of these signalling pathways could lead towards developing rational therapeutic strategies aimed to reduce cell death during a clostridialmyonecrosis.

Cellular and subcellular localization of PKMζ

In contrast to protein kinases that participate in long-term potentiation (LTP) induction and memory consolidation, the autonomously active atypical protein kinase C isoform, protein kinase Mzeta (PKMζ), functions in the core molecular mechanism of LTP maintenance and long-term memory storage. Here, using multiple complementary techniques for light and electron microscopic immunolocalization, we present the first detailed characterization of the cellular and subcellular distribution of PKMζ in rat hippocampus and neocortex. We find that PKMζ is widely expressed in forebrain with prominent immunostaining in hippocampal and neocortical grey matter, and weak label in white matter. In hippocampal and cortical pyramidal cells, PKMζ expression is predominantly somatodendritic, and electron microscopy highlights the kinase at postsynaptic densities and in clusters within spines. In addition, nuclear label and striking punctate immunopositive structures in a paranuclear and dendritic distribution are seen by confocal microscopy, occasionally at dendritic bifurcations. PKMζ immunoreactive granules are observed by electron microscopy in cell bodies and dendrites, including endoplasmic reticulum. The widespread distribution of PKMζ in nuclei, nucleoli and endoplasmic reticulum suggests potential roles of this kinase in cell-wide mechanisms involving gene expression, biogenesis of ribosomes and new protein synthesis. The localization of PKMζ within postsynaptic densities and spines suggests sites where the kinase stores information during LTP maintenance and long-term memory.

A cis-element with mixed G-quadruplex structure of NPGPx promoter is essential for nucleolin-mediated transactivation on non-targeting siRNA stress

We reported that non-targeting siRNA (NT-siRNA) stress induces non-selenocysteine containing phospholipid hydroperoxide glutathione peroxidase (NPGPx) expression to cooperate with exoribonuclease XRN2 for releasing the stress [Wei,P.C., Lo,W.T., Su,M.I., Shew,J.Y. and Lee,W.H. (2011) Non-targeting siRNA induces NPGPx expression to cooperate with exoribonuclease XRN2 for releasing the stress. Nucleic Acids Res., 40, 323-332]. However, how NT-siRNA stress inducing NPGPx expression remains elusive. In this communication, we showed that the proximal promoter of NPGPx contained a mixed G-quadruplex (G4) structure, and disrupting the structure diminished NT-siRNA induced NPGPx promoter activity. We also demonstrated that nucleolin (NCL) specifically bonded to the G4-containing sequences to replace the originally bound Sp1 at the NPGPx promoter on NT-siRNA stress. Consistently, overexpression of NCL further increased NPGPx promoter activity, whereas depletion of NCL desensitized NPGPx promoter to NT-siRNA stress. These results suggest that the cis-element with mixed G4 structure at the NPGPx promoter plays an essential role for its transactivation mediated by NCL to release cells from NT-siRNA stress.

PIKE mediates EGFR proliferative signaling in squamous cell carcinoma cells

One of the key drivers for squamous cell carcinoma (SCC) proliferation is activation of the epidermal growth factor receptor (EGFR), a known proto-oncogene. However, the mechanism of EGFR-dependent SCC proliferation remains unclear. Our previous studies indicate that epidermal growth factor (EGF)-induced SCC cell proliferation requires the SH3 domain of phospholipase C-γ1 (PLC-γ1), but not its catalytic activity. The SH3 domain of PLC-γ1 is known to activate the short form of nuclear phosphatidylinositol 3-kinase enhancer (PIKE) that enhances the activity of nuclear class Ia phosphatidylinositol 3-kinase (PI3K) required for proliferation. However, PIKE has been described for more than a decade to be present exclusively in neuronal cells. In the present study, we found that PIKE was highly expressed in malignant human keratinocytes (SCC4 and SCC12B2) but had low expression in normal human keratinocytes. Immunohistochemical analysis showed strong nuclear staining of PIKE in human epidermal and tongue SCC specimens but little staining in the adjacent non-cancerous epithelium. Treatment of SCC4 cells with EGF-induced translocation of PLC-γ1 to the nucleus and binding of PLC-γ1 to the nuclear PIKE. Knockdown of PLC-γ1 or PIKE blocked EGF-induced activation of class Ia PI3K and protein kinase C-ζ and phosphorylation of nucleolin in the nucleus as well as EGF-induced SCC cell proliferation. However, inhibition of the catalytic activity of PLC-γ1 had little effect. These data suggest that PIKE has a critical role in EGF-induced SCC cell proliferation and may function as a proto-oncogene in SCC.

Novel atypical PKC inhibitors prevent vascular endothelial growth factor-induced blood-retinal barrier dysfunction

Pro-inflammatory cytokines and growth factors such as VEGF (vascular endothelial growth factor) contribute to the loss of the BRB (blood-retinal barrier) and subsequent macular oedema in various retinal pathologies. VEGF signalling requires PKCβ [conventional PKC (protein kinase C)] activity; however, PKCβ inhibition only partially prevents VEGF-induced endothelial permeability and does not affect pro-inflammatory cytokine-induced permeability, suggesting the involvement of alternative signalling pathways. In the present study, we provide evidence for the involvement of aPKC (atypical PKC) signalling in VEGF-induced endothelial permeability and identify a novel class of inhibitors of aPKC that prevent BRB breakdown in vivo. Genetic and pharmacological manipulations of aPKC isoforms were used to assess their contribution to endothelial permeability in culture. A chemical library was screened using an in vitro kinase assay to identify novel small-molecule inhibitors, and further medicinal chemistry was performed to delineate a novel pharmacophore. We demonstrate that aPKC isoforms are both sufficient and required for VEGF-induced endothelial permeability. Furthermore, these specific, potent, non-competitive, small-molecule inhibitors prevented VEGF-induced tight junction internalization and retinal endothelial permeability in response to VEGF in both primary culture and in rodent retina. The results of the present study suggest that aPKC inhibition with 2-amino-4-phenyl-thiophene derivatives may be developed to preserve the BRB in retinal diseases such as diabetic retinopathy or uveitis, and the BBB (blood-brain barrier) in the presence of brain tumours.

RNA-binding protein nucleolin in disease

Nucleolin is a multifunctional protein localized primarily in the nucleolus, but also found in the nucleoplasm, cytoplasm and cell membrane. It is involved in several aspects of DNA metabolism, and participates extensively in RNA regulatory mechanisms, including transcription, ribosome assembly, mRNA stability and translation, and microRNA processing. Nucleolin's implication in disease is linked to its ability to associate with target RNAs via its four RNA-binding domains and its arginine/glycin-rich domain. By modulating the post-transcriptional fate of target mRNAs, which typically bear AU-rich and/or G-rich elements, nucleolin has been linked to cellular events that influence disease, notably cell proliferation and protection against apoptotic death. Through its diverse RNA functions, nucleolin is increasingly implicated in pathological processes, particularly cancer and viral infection. Here, we review the RNA-binding activities of nucleolin, its influence on gene expression patterns, and its impact upon diseases. We also discuss the rising interest in targeting nucleolin therapeutically.

Glycosilated nucleolin as marker for human gliomas

Nucleolin is a multifunctional DNA and RNA binding protein involved in regulation of gene transcription, chromatin remodeling, RNA metabolism, and ribosomal RNA synthesis. Nucleolin seems to be over-expressed in highly proliferative cells and is involved in many aspect of gene expression: DNA recombination and replication, RNA transcription by RNA polymerase I and II, rRNA processing, mRNA stabilization, cytokinesis, and apoptosis. Although nucleolin is localized predominantly in the nucleolus, it has also been shown to be localized in a phosphorylated/glycolsilated form on the cell surface of different cells. Numerous articles dealing with surface nucleolin targeting for tumor therapy have been recently published. However, at present, no extensive informations are so far available for the presence of nucleolin in human gliomas. In the present work we investigated on the presence and localization of nucleolin in glioma on glioma specimens at different grade of malignancy and on primary glioma cell cultures derived by surgical resection, trying to correlate the presence of glycosilated membrane nucleolin with the malignancy grade. To this purpose an antibody produced by us against gp273 protein, demonstrated to recognized the glycosilated surface nucleolin, has been used. The results obtained demonstrate that surface nucleolin increase with the malignancy grade thus suggesting that it may constitute a histopathological marker for glioma grading and a possible tool for targeted therapy.

Heat shock protein 90 stabilizes nucleolin to increase mRNA stability in mitosis

Most studies on heat shock protein 90 (Hsp90) have focused on the involvement of Hsp90 in the interphase, whereas the role of this protein in the nucleus during mitosis remains largely unclear. In this study, we found that the level of the acetylated form of Hsp90 decreased dramatically during mitosis, which indicates more chaperone activity during mitosis. We thus probed proteins that interacted with Hsp90 by liquid chromatography/mass spectrometry (LC/MS) and found that nucleolin was one of those interacting proteins during mitosis. The nucleolin level decreased upon geldanamycin treatment, and Hsp90 maintained the cyclin-dependent kinase 1 (CDK1) activity to phosphorylate nucleolin at Thr-641/707. Mutation of Thr-641/707 resulted in the destabilization of nucleolin in mitosis. We globally screened the level of mitotic mRNAs and found that 229 mRNAs decreased during mitosis in the presence of geldanamycin. Furthermore, a bioinformatics tool and an RNA immunoprecipitation assay found that 16 mRNAs, including cadherin and Bcl-xl, were stabilized through the recruitment of nucleolin to the 3'-untranslated regions (3'-UTRs) of those genes. Overall, strong correlations exist between the up-regulation of Hsp90, nucleolin, and the mRNAs related to tumorigenesis of the lung. Our findings thus indicate that nucleolin stabilized by Hsp90 contributes to the lung tumorigenesis by increasing the level of many tumor-related mRNAs during mitosis.

PI 3-kinase and PKCζ mediate insulin-induced potentiation of NMDA receptor currents in Xenopus oocytes

Insulin modulates N-methyl-d-aspartate (NMDA) receptors in the CNS and potentiates recombinant NMDA receptor currents in Xenopus oocytes. We have previously found that insulin's potentiation of NMDA receptor currents in oocytes occurs in a subunit specific manner and via phosphorylation of specific C-terminal sites by protein tyrosine kinases (PTKs) and C-type protein kinases (PKCs). Insulin-mediated current potentiation of receptors containing the NR2A subunit occurs solely through the activation of PKCs. Activation of phosphoinositide 3-kinase (PI 3-kinase) is known to trigger many insulin-stimulated signaling pathways, and we show here that it lies at a critical step in the insulin-mediated potentiation of NMDA receptor currents. Incubation with the PI 3-kinase inhibitor wortmannin eliminates insulin potentiation of NMDA receptor currents in the oocytes. Atypical isoforms of PKC are known to be activated downstream in the insulin signaling pathway via activation of PI 3-kinase. We demonstrate that the atypical isoform PKC zeta (PKCζ) has a role in insulin-stimulated current potentiation of NR2A-containing NMDA receptors using an isoform-specific pseudosubstrate inhibitor of PKCζ.

Glutathione peroxidase-1 in health and disease: from molecular mechanisms to therapeutic opportunities

Reactive oxygen species, such as superoxide and hydrogen peroxide, are generated in all cells by mitochondrial and enzymatic sources. Left unchecked, these reactive species can cause oxidative damage to DNA, proteins, and membrane lipids. Glutathione peroxidase-1 (GPx-1) is an intracellular antioxidant enzyme that enzymatically reduces hydrogen peroxide to water to limit its harmful effects. Certain reactive oxygen species, such as hydrogen peroxide, are also essential for growth factor-mediated signal transduction, mitochondrial function, and maintenance of normal thiol redox-balance. Thus, by limiting hydrogen peroxide accumulation, GPx-1 also modulates these processes. This review explores the molecular mechanisms involved in regulating the expression and function of GPx-1, with an emphasis on the role of GPx-1 in modulating cellular oxidant stress and redox-mediated responses. As a selenocysteine-containing enzyme, GPx-1 expression is subject to unique forms of regulation involving the trace mineral selenium and selenocysteine incorporation during translation. In addition, GPx-1 has been implicated in the development and prevention of many common and complex diseases, including cancer and cardiovascular disease. This review discusses the role of GPx-1 in these diseases and speculates on potential future therapies to harness the beneficial effects of this ubiquitous antioxidant enzyme.

Enhanced translation by Nucleolin via G-rich elements in coding and non-coding regions of target mRNAs

RNA-binding proteins (RBPs) regulate gene expression at many post-transcriptional levels, including mRNA stability and translation. The RBP nucleolin, with four RNA-recognition motifs, has been implicated in cell proliferation, carcinogenesis and viral infection. However, the subset of nucleolin target mRNAs and the influence of nucleolin on their expression had not been studied at a transcriptome-wide level. Here, we globally identified nucleolin target transcripts, many of which encoded cell growth- and cancer-related proteins, and used them to find a signature motif on nucleolin target mRNAs. Surprisingly, this motif was very rich in G residues and was not only found in the 3'-untranslated region (UTR), but also in the coding region (CR) and 5'-UTR. Nucleolin enhanced the translation of mRNAs bearing the G-rich motif, since silencing nucleolin did not change target mRNA stability, but decreased the size of polysomes forming on target transcripts and lowered the abundance of the encoded proteins. In summary, nucleolin binds G-rich sequences in the CR and UTRs of target mRNAs, many of which encode cancer proteins, and enhances their translation.

PKC isoforms interact with and phosphorylate DNMT1

Background

DNA methyltransferase 1 (DNMT1) has been shown to be phosphorylated on multiple serine and threonine residues, based on cell type and physiological conditions. Although recent studies have suggested that protein kinase C (PKC) may be involved, the individual contribution of PKC isoforms in their ability to phosphorylate DNMT1 remains unknown. The PKC family consists of at least 12 isoforms that possess distinct differences in structure, substrate requirement, expression and localization.

Results

Here we show that PKCα, βI, βII, δ, γ, η, ζ and μ preferentially phosphorylate the N-terminal domain of human DNMT1. No such phosphorylation of DNMT1 was observed with PKCε. Using PKCζ as a prototype model, we also found that PKC physically interacts with and phosphorylates DNMT1. In vitro phosphorylation assays conducted with recombinant fragments of DNMT1 showed that PKCζ preferentially phosphorylated the N-terminal region of DNMT1. The interaction of PKCζ with DNMT1 was confirmed by GST pull-down and co-immunoprecipitation experiments. Co-localization experiments by fluorescent microscopy further showed that endogenous PKCζ and DNMT1 were present in the same molecular complex. Endogenous PKCζ activity was also detected when DNMT1 was immunoprecipitated from HEK-293 cells. Overexpression of both PKCζ and DNMT1 in HEK-293 cells, but not of either alone, reduced the methylation status of genes distributed across the genome. Moreover, in vitro phosphorylation of DNMT1 by PKCζ reduced its methytransferase activity.

Conclusions

Our results indicate that phosphorylation of human DNMT1 by PKC is isoform-specific and provides the first evidence of cooperation between PKCζ and DNMT1 in the control of the DNA methylation patterns of the genome.

Targeting neural-restrictive silencer factor sensitizes tumor cells to antibody-based cancer immunotherapy in vitro via multiple mechanisms

Tumor cells escape clearance by complement by abundantly expressing CD59 and other membrane complement regulators. Recently, we designed a peptide derived from the neural-restrictive silencer factor (REST), REST68, which we showed to inhibit expression of CD59 in tumors lacking the full-length REST and proposed a detailed model for regulation of CD59 expression via interplay between REST and nucleolin (NCL) transcription factors. In this paper, we study in detail the mechanisms for sensitization of malignant cells to Ab-based cancer immunotherapy by the REST68 peptide and the implications of the REST/NCL model for the design of treatment resulting in higher tumor susceptibility. REST68 inhibited CD59 expression in malignant cells expressing either truncated or full-length REST, but not in nonmalignant cells. However, activation of protein kinase C (PKC) in nonmalignant cells, a process that contributes to cellular transformation, phosphorylated NCL and enabled suppression of CD59 expression by the REST68. Combined treatment of different tumor types with REST68 and PKC inhibitor synergized to further suppress CD59 expression and reduce resistance to complement lysis. The combined treatment also increased susceptibility of tumors expressing either of the REST isoforms to PBMC-mediated killing, which, at least in part, accounted for the strong promotion of apoptosis by the REST68/PKC inhibitor. These data demonstrate that REST68 sensitizes tumors to Ab-based cancer immunotherapy via multiple mechanisms. Furthermore, the REST/NCL interplay model for regulation of expression of cd59 and other genes involved in cell survival enables the design of treatments for different tumor types to achieve more efficient tumor clearance.

Interplay between REST and nucleolin transcription factors: a key mechanism in the overexpression of genes upon increased phosphorylation

Non-malignant cells can be transformed via the activation of kinases that control degradation of neural-restrictive silencer factor (REST). Here, we identify a mechanism that contributes to the activation of genes, expression of which is controlled by responsive elements containing overlapping binding sites for REST and nucleolin. We demonstrate that both phosphorylated and non-phosphorylated nucleolin-bound DNA; however, only phosphorylated nucleolin successfully competed with either full-length REST or a REST-derived DNA-binding peptide, REST68, for binding to the overlapping binding sites. We show that this interplay between the two transcription factors regulates the activation of cell survival and immunomodulatory genes in tumors and non-malignant cells with activated protein kinase C, which is accompanied with alterations in cell proliferation and apoptosis. We propose a model for the regulation of these genes, which brings a new insight into the molecular mechanisms that control cellular transformation driven by activation of protein kinases.

Protein kinase Czeta regulates Cdk5/p25 signaling during myogenesis

Atypical protein kinase Cζ (PKCζ) is emerging as a mediator of differentiation. Here, we describe a critical role for PKCζ during myogenic differentiation. Our results identify PKCζ as a controller of myogenic differentiation by its regulation of Cdk5.

Atypical protein kinase Cζ (PKCζ) is emerging as a mediator of differentiation. Here, we describe a novel role for PKCζ in myogenic differentiation, demonstrating that PKCζ activity is indispensable for differentiation of both C2C12 and mouse primary myoblasts. PKCζ was found to be associated with and to regulate the Cdk5/p35 signaling complex, an essential factor for both neuronal and myogenic differentiation. Inhibition of PKCζ activity prevented both myotube formation and simultaneous reorganization of the nestin intermediate filament cytoskeleton, which is known to be regulated by Cdk5 during myogenesis. p35, the Cdk5 activator, was shown to be a specific phosphorylation target of PKCζ. PKCζ-mediated phosphorylation of Ser-33 on p35 promoted calpain-mediated cleavage of p35 to its more active and stable fragment, p25. Strikingly, both calpain activation and the calpain-mediated cleavage of p35 were shown to be PKCζ-dependent in differentiating myoblasts. Overall, our results identify PKCζ as a controller of myogenic differentiation by its regulation of the phosphorylation-dependent and calpain-mediated p35 cleavage, which is crucial for the amplification of the Cdk5 activity that is required during differentiation.

The apicobasal polarity kinase aPKC functions as a nuclear determinant and regulates cell proliferation and fate during Xenopus primary neurogenesis

During neurogenesis in Xenopus, apicobasally polarised superficial and non-polar deep cells take up different fates: deep cells become primary neurons while superficial cells stay as progenitors. It is not known whether the proteins that affect cell polarity also affect cell fate and how membrane polarity information may be transmitted to the nucleus. Here, we examine the role of the polarity components, apically enriched aPKC and basolateral Lgl2, in primary neurogenesis. We report that a membrane-tethered form of aPKC (aPKC-CAAX) suppresses primary neurogenesis and promotes cell proliferation. Unexpectedly, both endogenous aPKC and aPKC-CAAX show some nuclear localisation. A constitutively active aPKC fused to a nuclear localisation signal has the same phenotypic effect as aPKC-CAAX in that it suppresses neurogenesis and enhances proliferation. Conversely, inhibiting endogenous aPKC with a dominant-negative form that is restricted to the nucleus enhances primary neurogenesis. These observations suggest that aPKC has a function in the nucleus that is important for cell fate specification during primary neurogenesis. In a complementary experiment, overexpressing basolateral Lgl2 causes depolarisation and internalisation of superficial cells, which form ectopic neurons when supplemented with a proneural factor. These findings suggest that both aPKC and Lgl2 affect cell fate, but that aPKC is a nuclear determinant itself that might shuttle from the membrane to the nucleus to control cell proliferation and fate; loss of epithelial cell polarity by Lgl2 overexpression changes the position of the cells and is permissive for a change in cell fate.

Identification of a novel nucleolin related protein (NRP) gene expressed during rat spermatogenesis

Background

Nucleolin is a major nucleolar phosphoprotein involved in various steps of ribosome biogenesis in eukaryotic cells. As nucleolin plays a significant role in ribosomal RNA transcription we were interested in examining in detail the expression of nucleolin across different stages of spermatogenesis and correlate with the transcription status of ribosomal DNA in germ cells.

Results

By RT PCR and western blot analysis we found that nucleolin is strongly down regulated in meiotic spermatocytes and haploid germ cells. We have identified a new nucleolin related protein (NRP) gene in the rat genome, which is over expressed in the testis and is up regulated several fold in meiotic spermatocytes and haploid germ cells. The NRP protein lacks the acidic stretches in its N terminal domain, and it is encoded in rat chromosome 15 having a different genomic organization as compared to nucleolin gene present on chromosome 9. We have also found NRP genes encoded in genomes of other mammalian species. We performed run-on transcription assay where we have observed that rDNA is transcribed at much lower level in meiotic spermatocytes and haploid spermatids as compared to diploid cells. By siRNA knock down experiments we could also demonstrate that NRP can support rDNA transcription in the absence of nucleolin.

Conclusion

We have identified a new nucleolin variant over expressed in germ cells in rat and analyzed its domain structure. We attribute that the transcriptional activity of rDNA genes in the late spermatogenesis is due to the presence of this variant NRP. The expression of this variant in the germ cells in the absence of nucleolin, could have additional functions in the mammalian spermatogenesis which needs to be investigated further.

The atypical protein kinase C in Aplysia can form a protein kinase M by cleavage

In vertebrates, a brain-specific transcript from the atypical protein kinase C (PKC) zeta gene encodes protein kinase M (PKM) zeta, a constitutively active kinase implicated in the maintenance of synaptic plasticity and memory. We have cloned the atypical PKC from Aplysia, PKC Apl III. We did not find a transcript in Aplysia encoding PKMzeta, and evolutionary analysis of atypical PKCs suggests formation of this transcript is restricted to vertebrates. Instead, over-expression of PKC Apl III in Aplysia sensory neurons leads to production of a PKM fragment of PKC Apl III. This cleavage was induced by calcium and blocked by calpain inhibitors. Moreover, nervous system enriched spliced forms of PKC Apl III show enhanced cleavage. PKC Apl III could also be activated through phosphorylation downstream of phosphoinositide 3-kinase. We suggest that PKM forms of atypical PKCs play a conserved role in memory formation, but the mechanism of formation of these kinases has changed over evolution.

Cell polarity emerges at first cleavage in sea urchin embryos

In protostomes, cell polarity is present after fertilization whereas most deuterostome embryos show minimal polarity during the early cleavages. We now show establishment of cell polarity as early as the first cleavage division in sea urchin embryos. We find, using the apical markers G(M1), integrins, and the aPKC-PAR6 complex, that cells are polarized upon insertion of distinct basolateral membrane at the first division. This early apical-basolateral polarity, similar to that found in much larger cleaving amphibian zygotes, reflects precocious functional epithelial cell polarity. Isolated cleavage blastomeres exhibit polarized actin-dependent fluid phase endocytosis only on the G(M1), integrin, microvillus-containing apical surface. A role for a functional PAR complex in cleavage plane determination was shown with experiments interfering with aPKC activity, which results in several spindle defects and compromised blastula development. These studies suggest that cell and embryonic polarity is established at the first cleavage, mediated in part by the Par complex of proteins, and is achieved by directed insertion of basolateral membrane in the cleavage furrow.

Interruption of RNA processing machinery by a small compound, 1-[(4-chlorophenyl)methyl]-1H-indole-3-carboxaldehyde (oncrasin-1)

Protein kinase Ciota (PKCiota) is activated by oncogenic Ras proteins and is required for K-Ras-induced transformation and colonic carcinogenesis in vivo. However, the role of PKCiota in signal transduction and oncogenesis is not clear. We recently identified a small molecule, designated 1-[(4-chlorophenyl)methyl]-1H-indole-3-carboxaldehyde (oncrasin-1), that can selectively kill K-Ras mutant cancer cells and induce abnormal nuclear aggregation of PKCiota in sensitive cells but not in resistant cells. To determine the causes and biological consequences of PKCiota aggregates in the nucleus, we analyzed the effect of oncrasin-1 on proteins involved in DNA repair and RNA processing. Our results showed that oncrasin-1 treatment led to coaggregation of PKCiota and splicing factors into megaspliceosomes but had no obvious effects on the DNA repair molecule Rad51. Moreover, oncrasin-1 treatment suppressed the phosphorylation of the largest subunit of RNA polymerase II and the expression of intronless reporter genes in sensitive cells but not in resistant cells, suggesting that suppression of RNA transcription is a major effect of oncrasin-1 treatment. Studies with cultured cells or with recombinant proteins showed that oncrasin-1 can disrupt the interaction of PKCiota and cyclin-dependent protein kinase 9/cyclin T1 complex, which is known to phosphorylate the largest subunit of RNA polymerase II and is required for RNA transcription. Together, our results suggest that oncrasin-1 suppresses the function of RNA processing machinery and that PKCiota might be involved in the biological function of RNA processing complexes.

The cell surface expressed nucleolin is a glycoprotein that triggers calcium entry into mammalian cells

Nucleolin is an ubiquitous nucleolar phosphoprotein involved in fundamental aspects of transcription regulation, cell proliferation and growth. It has also been described as a shuttling molecule between nucleus, cytosol and the cell surface. Several studies have demonstrated that surface nucleolin serves as a receptor for various extracellular ligands implicated in cell proliferation, differentiation, adhesion, mitogenesis and angiogenesis. Previously, we reported that nucleolin in the extranuclear cell compartment is a glycoprotein containing N- and O-glycans. In the present study, we show that glycosylation is an essential requirement for surface nucleolin expression, since it is prevented when cells are cultured in the presence of tunicamycin, an inhibitor of N-glycosylation. Accordingly, surface but not nuclear nucleolin is radioactively labeled upon metabolic labeling of cells with [(3)H]glucosamine. Besides its well-demonstrated role in the internalization of specific ligands, here we show that ligand binding to surface nucleolin could also induce Ca(2+) entry into cells. Indeed, by flow cytometry, microscopy and patch-clamp experiments, we show that the HB-19 pseudopeptide, which binds specifically surface nucleolin, triggers rapid and intense membrane Ca(2+) fluxes in various types of cells. The use of several drugs then indicated that Store-Operated Ca(2+) Entry (SOCE)-like channels are involved in the generation of these fluxes. Taken together, our findings suggest that binding of an extracellular ligand to surface nucleolin could be involved in the activation of signaling pathways by promoting Ca(2+) entry into cells.

Nuclear localization of the zebrafish tight junction protein nagie oko

The tight junctions-associated MAGUK protein nagie oko is closely related to Drosophila Stardust, mouse protein associated with lin-seven 1 (Pals1), and human MAGUK p55 subfamily member 5 (Mpp5). As a component of the evolutionarily conserved Crumbs protein complex, nagie oko is essential for the maintenance of epithelial cell polarity. Here, we show that nagie oko contains a predicted nuclear export and two conserved nuclear localization signals. We find that loss of the predicted nuclear export signal results in nuclear protein accumulation. We show that nagie oko nuclear import is redundantly controlled by the two nuclear localization signals and the evolutionarily conserved region 1 (ECR1), which links nagie oko with Par6-aPKC. Finally, deletion forms of nagie oko that lack nuclear import and export signals complement several nagie oko mutant defects in cell polarity and epithelial integrity. This finding provides an entry point to potentially novel and unknown roles of this important cell polarity regulator.

Phosphatidylinositol-3-kinase-atypical protein kinase C signaling is required for Wnt attraction and anterior-posterior axon guidance

Wnt proteins are conserved axon guidance cues that control growth cone navigation. However, the intracellular signaling mechanisms that mediate growth cone turning in response to Wnts are unknown. We previously showed that Wnt-Frizzled signaling directs spinal cord commissural axons to turn anteriorly after midline crossing through an attractive mechanism. Here we show that atypical protein kinase C (aPKC), is required for Wnt-mediated attraction of commissural axons and proper anterior-posterior (A-P) pathfinding. A PKCzeta pseudosubstrate, a specific blocker of aPKC activity, and expression of a kinase-defective PKCzeta mutant in commissural neurons resulted in A-P randomization in "open-book" explants. Upstream of PKCzeta, heterotrimeric G-proteins and phosphatidylinositol-3-kinases (PI3Ks), are also required for A-P guidance, because pertussis toxin, wortmannin, and expression of a p110gamma kinase-defective construct all resulted in A-P randomization. Overexpression of p110gamma, the catalytic subunit of PI3Kgamma, caused precocious anterior turning of commissural axons before midline crossing in open-book explants and caused dissociated precrossing commissural axons, which are normally insensitive to Wnt attraction, to turn toward Wnt4-expressing cells. Therefore, we propose that atypical PKC signaling is required for Wnt-mediated A-P axon guidance and that PI3K can act as a switch to activate Wnt responsiveness during midline crossing.

The role of PKCzeta in NMDA-induced retinal ganglion cell death: prevention by aspirin

Intravitreal NMDA injection has been shown to induce the excitotoxic loss of retinal cells. The retinal ganglion cell apoptosis induced by NMDA is thought to play an important role in retinal ischemia injury and NMDA-injected rat has been used as a model of neuronal loss in diseases such as glaucoma. In this experimental model, we studied the early effects of NMDA leading to the degeneration of retinal ganglion cells. PKCzeta regulates the NF-kappaB pathway in cellular responses to various stresses and we have shown that aspirin inhibits purified human PKCzeta. We therefore investigated the molecular mechanism by which retinal cells limit ocular injury following NMDA treatment. We found that the NMDA-induced apoptosis of ganglion cells was mediated, at least partly, by PKCzeta. This enzyme was activated early in the cellular response to NMDA. Prolonged activation was followed by PKCzeta cleavage, and nuclear translocation of the C-terminal region of this protein-a critical event for the survival of retinal cells. We also found that pretreatment with aspirin or the coinjection of NMDA with a specific PKCzeta inhibitor counteracted the effects of NMDA. These findings provide new insight into the role played by PKCzeta in neuronal loss in glaucoma.

Protein kinase Czeta is required for oleic acid-induced secretion of glucagon-like peptide-1 by intestinal endocrine L cells

Long-chain, monounsaturated fatty acids (FAs) stimulate secretion of the incretin hormone, glucagon-like peptide-1 (GLP-1) from the intestinal L cell. Because the atypical protein kinase C (PKC), PKCzeta, is involved in FA signaling in many cells, the role of PKCzeta in FA-induced GLP-1 secretion was investigated, using the murine GLUTag L cell line and primary rat intestinal L cells. GLUTag cells expressed mRNA for several PKC isoforms, including PKCzeta, and PKCzeta protein was localized throughout the cytoplasm in GLUTag and primary L cells as well as normal mouse and rat L cells. Treatment with oleic acid (150-1000 microm) for 2 h increased GLP-1 secretion (P < 0.001), and this was abrogated by the PKCzeta inhibitor ZI (P < 0.05) and PKCzeta small interfering RNA transfection (P < 0.05) but not inhibition of classical/novel PKC isoforms. Although most PKCzeta was localized in the particulate compartment of GLUTag cells, oleate treatment did not alter PKCzeta levels or activity in this cell fraction. GLUTag cells expressed mRNA for the Gq-coupled FA receptor GPR120; however, oleic acid did not induce any changes in Akt, MAPK, or calcium, and pretreatment with LY294002 and PD98059 to inhibit phosphatidylinositol 3-kinase and MAPK, respectively, did not prevent the effects of oleic acid. Finally, GLUTag cells also released GLP-1 in response to arachidonic acid (P < 0.001) but were not affected by other long-chain FAs. These findings demonstrate that PKCzeta is required for oleic acid-induced GLP-1 secretion. This enzyme may therefore serve as a therapeutic target to enhance GLP-1 release in type 2 diabetes.

Bisindolylmaleimide I suppresses fibroblast growth factor-mediated activation of Erk MAP kinase in chondrocytes by preventing Shp2 association with the Frs2 and Gab1 adaptor proteins

Fibroblast growth factors (FGFs) inhibit chondrocyte proliferation via the Erk MAP kinase pathway. Here, we explored the role of protein kinase C in FGF signaling in chondrocytes. Erk activity in FGF2-treated RCS (rat chondrosarcoma) chondrocytes or human primary chondrocytes was abolished by the protein kinase C inhibitor bisindolylmaleimide I (Bis I). Bis I inhibited FGF2-induced activation of MEK, Raf-1, and Ras members of Erk signaling module but not the FGF2-induced tyrosine phosphorylation of Frs2 or the kinase activity of FGFR3, demonstrating that it targets the Erk cascade immediately upstream of Ras. Indeed, Bis I abolished the FGF2-mediated association of Shp2 tyrosine phosphatase with Frs2 and Gab1 adaptor proteins necessary for proper Ras activation. We also determined which PKC isoform is involved in FGF2-mediated activation of Erk. When both conventional and novel PKCs expressed by RCS chondrocytes (PKCalpha, -gamma, -delta, and -epsilon) were down-regulated by phorbol ester, cells remained responsive to FGF2 with Erk activation, and this activation was sensitive to Bis I. Moreover, treatment with PKClambda/zeta pseudosubstrate lead to significant reduction of FGF2-mediated activation of Erk, suggesting involvement of an atypical PKC.

Differential activation of protein kinase C isoforms following chemical ischemia in rat cerebral cortex slices

The aim of the current study was to characterize the effects of chemical ischemia and reperfusion at the transductional level in the brain. Protein kinase C isoforms (alpha, beta(1), beta(2), gamma, delta and epsilon) total levels and their distribution in the particulate and cytosolic compartments were investigated in superfused rat cerebral cortex slices: (i) under control conditions; (ii) immediately after a 5-min treatment with 10mM NaN(3), combined with 2mM 2-deoxyglucose (chemical ischemia); (iii) 1h after chemical ischemia (reperfusion). In control samples, all the PKC isoforms were detected; immediately after chemical ischemia, PKC beta(1), delta and epsilon isoforms total levels (cytosol+particulate) were increased by 2.9, 2.7 and 9.9 times, respectively, while alpha isoform was slightly reduced and gamma isoform was no longer detectable. After reperfusion, the changes displayed by alpha, beta(1), gamma, delta and epsilon were maintained and even potentiated, moreover, an increase in beta(2) (by 41+/-12%) total levels became significant. Chemical ischemia-induced a significant translocation to the particulate compartment of PKC alpha isoform, which following reperfusion was found only in the cytosol. PKC beta(1) and delta isoforms particulate levels were significantly higher both in ischemic and in reperfused samples than in the controls. Conversely, following reperfusion, PKC beta(2) and epsilon isoforms displayed a reduction in their particulate to total level ratios. The intracellular calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, 1mM, but not the N-methyl-d-asparate receptor antagonist, MK-801, 1muM, prevented the translocation of beta(1) isoform observed during ischemia. Both drugs were effective in counteracting reperfusion-induced changes in beta(2) and epsilon isoforms, suggesting the involvement of glutamate-induced calcium overload. These findings demonstrate that: (i) PKC isoforms participate differently in neurotoxicity/neuroprotection events; (ii) the changes observed following chemical ischemia are pharmacologically modulable; (iii) the protocol of in vitro chemical ischemia is suitable for drug screening.

ANG II-induced cell proliferation is dually mediated by c-Src/Yes/Fyn-regulated ERK1/2 activation in the cytoplasm and PKCζ-controlled ERK1/2 activity within the nucleus

High-affinity binding of angiotensin II (ANG II) to the ANG II type 1 receptor (AT1R) results in the activation of ERK1/2 mitogen-activated protein kinases (MAPK). However, the precise mechanism of ANG II-induced ERK1/2 activation has not been fully characterized. Here, we investigated the signaling events leading to ANG II-induced ERK1/2 activation using a c-Src/Yes/Fyn tyrosine kinase-deficient mouse embryonic fibroblast (MEF) cell line stably transfected with the AT1R (SYF/AT1). ERK1/2 activation was reduced by ∼50% within these cells compared with wild-type controls (WT/AT1). The remaining ∼50% of intracellular ERK1/2 activation was dependent upon heterotrimeric G protein and protein kinase C zeta (PKCζ) activation. Therefore, ANG II-induced ERK1/2 activation occurs via two independent mechanisms. We next investigated whether a loss of either c-Src/Yes/Fyn or PKCζ signaling affected ERK1/2 nuclear translocation and cell proliferation in response to ANG II. ANG II-induced cell proliferation was markedly reduced in SYF/AT1cells compared with WT/AT1cells ( P < 0.01), but interestingly, ERK2 nuclear translocation was normal. ANG II-induced nuclear translocation of ERK2 was blocked via pretreatment of WT/AT1cells with a PKCζ pseudosubstrate. ANG II-induced cell proliferation was significantly reduced in PKCζ pseudosubstrate-treated WT/AT1cells ( P < 0.01) and was completely blocked in SYF/AT1cells treated with this same compound. Thus ANG II-induced cell proliferation appears to be regulated by both ERK1/2-driven nuclear and cytoplasmic events. In response to ANG II, the ability of ERK1/2 to remain within the cytoplasm or translocate into the nucleus is controlled by c-Src/Yes/Fyn or heterotrimeric G protein/PKCζ signaling, respectively.

K-Ras4B proteins are expressed in the nucleolus: Interaction with nucleolin

Kirsten Ras4B (K-Ras4B) is a potent onco-protein that is expressed in the majority of human cell types and is frequently mutated in carcinomas. K-Ras4B, like other members of the Ras family of proteins, is considered to be a cytoplasmic protein that must be localized to the plasma membrane for activation. Here, using confocal microscopy and biochemical analysis, we show that K-Ras4B, but not H-Ras or the closely related K-Ras4A, is also present in the nucleoli of normal and transformed cells. Subcellular fractionation and immunostaining show that K-Ras4B is located not only in the cytoplasm, but also in the nucleolar compartment. Modification of a C-terminal hexa-lysine motif unique to K-Ras4B results in exclusively cytoplasmic forms of the protein. Nucleolin, a pleiotropic regulator of cellular processes, including transcriptional regulation, is also characterized by a nucleolar-like nuclear appearance. We show that K-Ras4B and nucleolin co-localize within the nucleus and that nucleolin physically associates with K-Ras4B. Inhibition of K-Ras4B/nucleolin association blocked nucleolar localization of K-Ras4B. Using siRNA to knockdown the expression of nucleolin eliminated the nucleolar localization of K-Ras4B and significantly repressed the activation of the well-characterized K-Ras4B transcriptional target Ap-1, but stimulated Elk1. These data provide evidence of a nucleolar localization of K-Ras4B and describe a functional association between K-Ras4B and nucleolin.

AGRO100 inhibits activation of nuclear factor-kappaB (NF-kappaB) by forming a complex with NF-kappaB essential modulator (NEMO) and nucleolin

AGRO100, also known as AS1411, is an experimental anticancer drug that recently entered human clinical trials. It is a member of a novel class of antiproliferative agents known as G-rich oligonucleotides (GRO), which are non-antisense, guanosine-rich phosphodiester oligodeoxynucleotides that form stable G-quadruplex structures. The biological activity of GROs results from their binding to specific cellular proteins as aptamers. One important target protein of GROs has been previously identified as nucleolin, a multifunctional protein expressed at high levels by cancer cells. Here, we report that AGRO100 also associates with nuclear factor-kappaB (NF-kappaB) essential modulator (NEMO), which is a regulatory subunit of the inhibitor of kappaB (IkappaB) kinase (IKK) complex, and also called IKKgamma. In the classic NF-kappaB pathway, the IKK complex is required for phosphorylation of IkappaBalpha and subsequent activation of the transcription factor NF-kappaB. We found that treatment of cancer cells with AGRO100 inhibits IKK activity and reduces phosphorylation of IkappaBalpha in response to tumor necrosis factor-alpha stimulation. Using a reporter gene assay, we showed that AGRO100 blocks both tumor necrosis factor-alpha-induced and constitutive NF-kappaB activity in human cancer cell lines derived from cervical, prostate, breast, and lung carcinomas. In addition, we showed that, in AGRO100-treated cancer cells, NEMO is coprecipitated by nucleolin, indicating that both proteins are present in the same complex. Our studies suggest that abrogation of NF-kappaB activity may contribute to the anticancer effects of AGRO100 and that nucleolin may play a previously unknown role in regulating the NF-kappaB pathway.

Nuclear Ptdlns(3,4,5)P3 signaling: an ongoing story

Phosphatidylinositol 3,4,5-trisphosphate (Ptdlns(3,4,5)P(3)) is linked to a variety of cellular functions, such as growth, cell survival, and differentiation. Ptdlns(3,4,5)P(3) is primarily synthesized by class I phosphoinositide 3-kinases and its hydrolysis by two 3-phosphoinositide 3-phosphatases, PTEN and SHIP proteins, leads to the production of two other second messengers, Ptdlns(4,5)P(2) and Ptdlns(3,4)P(2), respectively. Evidence accumulated over the last years strongly suggest that Ptdlns(3,4,5)P(3) is an important component of signaling pathway operating within the nucleus. Moreover, recent advances indicated that nuclear translocation of cell surface receptors could activate nuclear phosphoinositide 3-kinase suggesting a new mode of signal transduction. The aim of this review is intended to summarize the state of our knowledge on nuclear Ptdlns(3,4,5)P(3) and its metabolizing enzymes, and to highlight the emerging roles for intranuclear Ptdlns(3,4,5)P(3).

Phosphoinositide-specific phospholipase C (PI-PLC) β1 and nuclear lipid-dependent signaling

Over the last years, evidence has suggested that phosphoinositides, which are involved in the regulation of a large variety of cellular processes both in the cytoplasm and in the plasma membrane, are present also within the nucleus. A number of advances has resulted in the discovery that phosphoinositide-specific phospholipase C signalling in the nucleus is involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Even though nuclear inositol lipids hydrolysis generates second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. Among phosphoinositide-specific phospholipase C, the beta(1) isoform appears to be one of the key players of the nuclear lipid signaling. This review aims at highlighting the most significant and up-dated findings about phosphoinositide-specific phospholipase C beta(1) in the nucleus.

Phosphorylation of the spinach chloroplast 24 kDa RNA-binding protein (24RNP) increases its binding to petD and psbA 3' untranslated regions

The chloroplast 24 kDa RNA binding protein (24RNP) from Spinacea oleracea is a nuclear encoded protein that binds the 3' untranslated region (3'UTR) of some chloroplast mRNAs and seems to be involved in some processes of mRNA metabolism, such as 3'UTR processing, maturation and stabilization. The 24RNP is similar to the 28RNP which is involved in the correct maturation of petD and psbA 3'UTRs, and when phosphorylated, decreases its binding affinity for RNA. In the present work, we determined that the recombinant 24RNP was phosphorylated in vitro either by an animal protein kinase C, a plant Ca(2+)-dependent protein kinase, or a chloroplastic kinase activity present in a protein extract with 3'-end processing activity in which the 24RNP is also present. Phosphorylation of 24RNP increased the binding capacity (B(max)) 0.25 time for petD 3'UTR, and three times for psbA 3'UTR; the affinity for P-24RNP only increased when the interaction with petD was tested. Competition experiments suggested that B(max), not K(d), might be a more important factor in the P-24RNP-3'UTR interaction. The data suggested that the 24RNP role in chloroplast mRNA metabolism may be regulated in vivo by changes in its phosphorylation status carried out by a chloroplastic kinase.

Nuclear protein kinase C

Protein kinase C (PKC) isozymes constitute a family of ubiquitous phosphotransferases which act as key transducers in many agonist-induced signaling cascades. To date, at least 11 different PKC isotypes have been identified and are believed to play distinct regulatory roles. PKC isoforms are physiologically activated by a number of lipid cofactors. PKC is thought to reside in the cytoplasm in an inactive conformation and to translocate to the plasma membrane or cytoplasmic organelles upon cell activation by different stimuli. However, a sizable body of evidence collected over the last 20 years has shown PKC to be capable of translocating to the nucleus. Furthermore, PKC isoforms are resident within the nucleus. Studies from independent laboratories have to led to the identification of quite a few nuclear proteins which are PKC substrates and to the characterization of nuclear PKC-binding proteins which may be critical for finely tuning PKC function in this cell microenvironment. Several lines of evidence suggest that nuclear PKC isozymes are involved in the regulation of biological processes as important as cell proliferation and differentiation, gene expression, neoplastic transformation, and apoptosis. In this review, we shall highlight the most intriguing and updated findings about the functions of nuclear PKC isozymes.

Intranuclear 3'-phosphoinositide metabolism and Akt signaling: new mechanisms for tumorigenesis and protection against apoptosis?

Lipid second messengers, particularly those derived from the polyphosphoinositide metabolism, play a pivotal role in multiple cell signaling networks. Phosphoinositide 3-kinase (PI3K) generate 3'-phosphorylated inositol lipids that are key players in a multitude of cell functions. One of the best characterized targets of PI3K lipid products is the serine/threonine protein kinase Akt (protein kinase B, PKB). Recent findings have implicated the PI3K/Akt pathway in tumorigenesis because it stimulates cell proliferation and suppresses apoptosis. However, it was thought that this signal transduction network would exert its carcinogenetic effects mainly by operating in the cytoplasm. Evidence accumulated over the past 15 years has highlighted the presence of an autonomous nuclear inositol lipid cycle, and strongly suggests that lipid molecules are important components of signaling pathways operating at the nuclear level. PI3K, its lipid product phosphatidylinositol (3,4,5) trisphosphate (PtdIns(3,4,5)P3), and Akt have been identified within the nucleus and recent data suggest that they counteract apoptosis also by operating in this cell compartment through a block of caspase-activated DNase and inhibition of chromatin condensation. In this review, we shall summarize the most updated and intriguing findings about nuclear PI3K/PtdIns(3,4,5)P3/Akt in relationship with tumorigenesis and suppression of apoptotic stimuli.

In high glucose protein kinase C-zeta activation is required for mesangial cell generation of reactive oxygen species

BACKGROUND

We postulated that in mesangial cells exposed to high glucose, protein kinase C-zeta (PKC-zeta) is necessary for the generation of reactive oxygen species (ROS) by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and that the requirement of PKC-zeta for filamentous (F)-actin disassembly may involve ROS. To identify signaling mechanisms relevant to PKC-zeta activation and ROS generation, including phosphoinositide 3 kinase (PI3 kinase), we examined mesangial cell stimulation with platelet-derived growth factor (PDGF).

METHODS

In primary rat mesangial cells cultured in 5.6 mmol/L or 30 mmol/L d-glucose, PKC-zeta expression was identified with immunoblotting and activity was analyzed in cell membrane immunoprecipitates and by confocal immunofluorescence imaging. ROS generation was measured by dichlorofluorescein fluorescence using confocal microscopy and was inhibited by transfection of antisense against NADPH subunits p22(phox) or p47(phox) or with Tempol. F-actin disassembly was observed by dual-channel confocal fluorescence imaging. PI3 kinase activity was detected by immunoblotting of phosphorylated Akt.

RESULTS

In high glucose, generation of NADPH oxidase-dependent ROS was dependent on PKC-zeta. Conversely, sustained PKC-zeta activity was dependent on ROS generation, suggesting a positive feedback. PKC-zeta-dependent F-actin disassembly in high glucose required ROS generation. PDGF stimulated NADPH oxidase generation of ROS through a PKC-zeta mechanism that was independent of Akt phosphorylation and remained unchanged in high glucose.

CONCLUSION

In high glucose, mesangial cell PKC-zeta is required for ROS generation from NADPH oxidase similar to PDGF stimulation of PKC-zeta-dependent ROS generation through a pathway independent of PI3 kinase. F-actin disassembly in high glucose also requires ROS. A positive feedback loop occurs between ROS and the activation of PKC-zeta in high glucose.

Malondialdehyde-acetaldehyde adducts decrease bronchial epithelial wound repair

Most people who abuse alcohol are cigarette smokers. Previously, we have shown that malondialdehyde, an inflammation product of lipid peroxidation, and acetaldehyde, a component of both ethanol metabolism and cigarette smoke, form protein adducts that stimulate protein kinase C (PKC) activation in bronchial epithelial cells. We have also shown that PKC can regulate bronchial epithelial cell wound repair. We hypothesize that bovine serum albumin adducted with malondialdehyde and acetaldehyde (BSA-MAA) decreases bronchial epithelial cell wound repair via binding to scavenger receptors on bronchial epithelial cells. To test this, confluent monolayers of bovine bronchial epithelial cells were grown in serum-free media prior to wounding the cells. Bronchial epithelial cell wound closure was inhibited in a dose-dependent manner (up to 60%) in the presence of BSA-MAA than in media treated cells (Laboratory of Human Carcinogenesis [LHC]-9-Roswell Park Memorial Institute [RPMI]). The specific scavenger receptor ligand, fucoidan, also stimulated PKC activation and decreased wound repair. Pretreatment with fucoidan blocked malondialdehyde-acetaldehyde binding to bronchial epithelial cells. When bronchial epithelial cells were preincubated with a PKC alpha inhibitor, Gö 6976, the inhibition of wound closure by fucoidan and BSA-MAA was blocked. Western blot demonstrated the presence of several scavenger receptors on bronchial epithelial cell membranes, including SRA, SRBI, SRBII, and CD36. Scavenger receptor-mediated activation of PKC alpha may function to reduce wound healing under conditions of alcohol and cigarette smoke exposure where malondialdehyde-acetaldehyde adducts may be present.

Adenosine-Dependent Induction of Glutathione Peroxidase 1 in Human Primary Endothelial Cells and Protection Against Oxidative Stress

Cellular glutathione peroxidase (GPx-1), a selenocysteine-containing enzyme, plays a central role in protecting cells from oxidative injury. GPx-1 is ubiquitously expressed in eukaryotic cells where it reduces hydrogen and lipid peroxides to alcohols. Adenosine, which is released from stressed or injured cells, protects against ischemia/reperfusion injury and apoptosis. In this study, we hypothesize that the cytoprotective effect of adenosine involves an increase in the activity of GPx-1. Treatment of human primary pulmonary artery endothelial cells (HPAECs) with 50 μmol/L adenosine in the presence of 10 μmol/L erytho-9-(2-hydroxy-3-nonyl)adenine (EHNA), an adenosine deaminase inhibitor, for 48 hours increased GPx-1 mRNA levels 2-fold. GPx-1 protein and enzyme activity also increased ≈2-fold after treatment. The induction of GPx-1 expression was found to be a consequence of increased mRNA stability and not an increase in transcription. Bisindolylmaleimide I (BIM), a protein kinase C signaling pathway inhibitor, significantly attenuated the induction of GPx-1 mRNA by ≈36%. The adenosine/EHNA-treated cells were more resistant to hydrogen peroxide stress. Both pharmacological inhibition and siRNA knockdown of GPx-1 attenuated the protective affect of adenosine/EHNA treatment, indicating that the adenosine-induced increase in GPx-1 contributes to an increase in cellular protection against oxidative stress. These data suggest that adenosine may protect the cardiovascular system from ischemia/reperfusion injury, in part, by enhancing the expression of the central intracellular antioxidant enzyme, GPx-1.

Aspirin prevention of NMDA-induced neuronal death by direct protein kinase Cζ inhibition

Abstract Aspirin has been shown to protect against glutamate neurotoxicity via the nuclear factor kappaB pathway. Some studies have implicated the atypical protein kinase C (PKC) zeta (zeta) isoform in cell protection, but the mechanism involved remains unclear. We show here that aspirin exerts at least some of its effects through PKCzeta, decreasing the NMDA-induced activation, cleavage and nuclear translocation of this molecule. Aspirin (acetylsalicylic acid) directly inhibited the protein kinase activity of PKCzeta, whereas salicylic acid did not. This direct effect of aspirin on purified human PKCzeta is consistent with PKCzeta inhibition preventing the NMDA-induced death of cortical neurones. Caspase-3 inhibition blocked the cleavage and nuclear translocation of PKCzeta, whereas caspase-1-inhibition did not. Thus, PKCzeta (protein kinase Mzeta) regulates nuclear events essential for the initiation of the apoptotic pathway. Aspirin protects cells against NMDA-induced apoptosis by means of a novel mechanism targeting PKCzeta, a key molecule in inflammatory responses and neurodegeneration.

Oleate promotes the proliferation of breast cancer cells via the G protein-coupled receptor GPR40

Evidence from epidemiological studies and animal models suggests a link between high levels of dietary fat intake and risk of breast cancer. In addition, obesity, in which circulating lipids are elevated, is associated with increased risk of various cancers. Relative to this point, we previously showed that oleate stimulates the proliferation of breast cancer cells and that phosphatidylinositol 3-kinase plays a role in this process. Nonetheless, questions remain regarding the precise mechanism(s) by which oleate promotes breast cancer cell growth. Pharmacological inhibitors of the GTP-binding proteins G(i)/G(o), phospholipase C, Src, and mitogenic-extracellular signal-regulated kinase 1/2 (MEK 1/2) decreased oleate-induced [3H]thymidine incorporation in the breast cancer cell line MDA-MB-231. In addition, oleate caused a rapid and transient rise in cytosolic Ca2+ and an increase in protein kinase B phosphorylation. Overexpressing in these cells the G protein-coupled receptor GPR40, a fatty acid receptor, amplified oleate-induced proliferation, whereas silencing the GPR40 gene using RNA interference decreased it. Overexpressing GPR40 in T47D and MCF-7 breast cancer cells that are poorly responsive to oleate allowed a robust proliferative action of oleate. The data indicate that the phospholipase C, MEK 1/2, Src, and phosphatidylinositol 3-kinase/protein kinase B signaling pathways are implicated in the proliferative signal induced by oleate and that these effects are mediated at least in part via the G protein-coupled receptor GPR40. The results suggest that GPR40 is implicated in the control of breast cancer cell growth by fatty acids and that GPR40 may provide a link between fat and cancer.

Intimin types alpha, beta, and gamma bind to nucleolin with equivalent affinity but lower avidity than to the translocated intimin receptor

The outer membrane adhesins of enteropathogenic Escherichia coli, Citrobacter rodentium, and enterohemorrhagic E. coli (EHEC) O157:H7 that mediate attach and efface intestinal lesions are classified as intimin alpha, beta, and gamma, respectively. Each of these intimin types binds to its cognate, bacterially encoded receptor (called Tir for translocated intimin receptor) to promote tight adherence of the organism to the host-cell plasma membrane. We previously reported that gamma intimin of EHEC O157:H7 also bound to a eucaryotic receptor that we determined was nucleolin. The objective of this study was to investigate in vitro and in vivo the interactions of intimins alpha, beta, and gamma with nucleolin in the presence of Tir from EHEC O157:H7. Protein binding experiments demonstrated that intimin of types alpha, beta, and gamma bound nucleolin with similar affinity. Moreover, all three intimin types co-localized with regions of nucleolin expressed on the surface of HEp-2 cells. When intimin alpha, beta, or gamma bound to Tir in vitro, the intimin interaction with nucleolin was blocked. Both Tir and nucleolin accumulated beneath intimin-presenting bacteria that had attached to the surface of HEp-2 cells. Taken together, these findings suggest that nucleolin is involved in bacterial adherence promoted by all intimin types and that Tir and nucleolin compete for intimin during adherence.

Identification of nucleolin as an AU-rich element binding protein involved in bcl-2 mRNA stabilization

bcl-2 mRNA contains an AU-rich element (ARE) that functions in regulating bcl-2 stability. Our earlier studies indicated that taxol- or okadaic acid-induced bcl-2 mRNA destabilization in HL-60 cells is associated with decreased binding of trans-acting factors to the ARE. To identify factors that play a role in the regulation of bcl-2 mRNA stability, bcl-2 ARE-binding proteins were purified from HL-60 cells. Three polypeptides of 100, 70, and 32 kDa were isolated from a bcl-2 ARE affinity matrix. Matrix-assisted laser desorption ionization mass spectroscopy analysis identified these proteins as full-length nucleolin and proteolytic fragments of nucleolin. RNA gel shifts assays indicated that recombinant nucleolin (residues 284-707) binds specifically to bcl-2 ARE RNA. In addition, recombinant nucleolin decreases the rate of decay of mRNA in HL-60 cell extracts in an ARE-dependent manner. Taxol or okadaic acid treatment of HL-60 cells results in proteolysis of nucleolin in a similar time frame as drug-induced bcl-2 mRNA down-regulation. These findings suggest that nucleolin functions as a bcl-2-stabilizing factor and that taxol and okadaic acid treatment induces apoptosis in HL-60 cells through a process that involves down-regulation of nucleolin and destabilization of bcl-2 mRNA.