Binding of polyamines to an autonomous domain of the regulatory subunit of protein kinase CK2 induces a conformational change in the holoenzyme. A proposed role for the kinase stimulation

VISTA promotes the metabolism and differentiation of myeloid-derived suppressor cells by STAT3 and polyamine-dependent mechanisms

Myeloid-derived suppressor cells (MDSCs) impair antitumor immune responses. Identifying regulatory circuits during MDSC development may bring new opportunities for therapeutic interventions. We report that the V-domain suppressor of T cell activation (VISTA) functions as a key enabler of MDSC differentiation. VISTA deficiency reduced STAT3 activation and STAT3-dependent production of polyamines, which causally impaired mitochondrial respiration and MDSC expansion. In both mixed bone marrow (BM) chimera mice and myeloid-specific VISTA conditional knockout mice, VISTA deficiency significantly reduced tumor-associated MDSCs but expanded monocyte-derived dendritic cells (DCs) and enhanced T cell-mediated tumor control. Correlated expression of VISTA and arginase-1 (ARG1), a key enzyme supporting polyamine biosynthesis, was observed in multiple human cancer types. In human endometrial cancer, co-expression of VISTA and ARG1 on tumor-associated myeloid cells is associated with poor survival. Taken together, these findings unveil the VISTA/polyamine axis as a central regulator of MDSC differentiation and warrant therapeutically targeting this axis for cancer immunotherapy.

Influence of CK2 protein kinase activity on the interaction between Trypanosoma cruzi and its vertebrate and invertebrate hosts

Chagas disease, endemic from Latin America, is caused by Trypanosoma cruzi and is transmitted by triatomine feces. This parasite undergoes complex morphological changes through its life cycle, promoted by significant changes in signal transduction pathways. The activity of protein kinase CK2 has been described in trypanosomatids. Using a specific peptide and radioactive ATP, we identified CK2 activity on the cellular surface and the cytoplasmic content in Trypanosoma cruzi, apart from the secreted form. Dephosphorylated casein promoted an increase of 48% in the secreted CK2 activity. Total extract of peritoneal macrophages from BALB/c and inactivated human serum promoted an increase of 67% and 36%, respectively, in this activity. The protein secreted by parasites was purified by HPLC and had shown compatibility with the catalytic subunit of mammalian CK2. Incubation of the parasites with CK2 inhibitors, added to the culture medium, prevented their growth. The opposite was observed when CK2 activators were used. Results of interaction between Trypanosoma cruzi and the gut of the vector have revealed that, in the presence of CK2 inhibitors, there is a reduction in the association rate. A similar inhibition profile was seen in the Trypanosoma cruzi-macrophages interaction, confirming the importance of this enzyme in the life cycle of this protozoan.

SpkH (Sll0005) from Synechocystis sp. PCC 6803 is an active Mn2+-dependent Ser kinase

Twelve genes for the potential serine-threonine protein kinases (STPKs) have been annotated in the genome of Synechocystis sp. PCC 6803. Based on similarities and distinctive domain organization, they were divided into two clusters: serine/threonine-protein N2-like kinases (PKN2-type) and "activity of bc1 complex" kinases (ABC1-type). While the activity of the PKN2-type kinases have been demonstrated, no ABC1-type kinases activity have hitherto been reported. In this study, a recombinant protein previously annotated as a potential STPK of ABC1-type (SpkH, Sll0005) was expressed and purified to homogeneity. We demonstrated SpkH phosphorylating activity and substrate preference for casein in in vitro assays using [γ-32P]ATP. Detailed analyses of activity showed that Mn2+ had the strongest activation effect. The activity of SpkH was significantly inhibited by heparin and spermine, but not by staurosporine. By means of semi-quantitative mass-spectrometric detection of phosphopeptides, we identified a consensus motif recognized by this kinase - X1X2pSX3E. Thus, we first report here that SpkH of Synechocystis represents a true active serine protein kinase, which shares the properties of casein kinases according to its substrate specificity and sensitivity to some activity effectors.

TTT (Tel2-Tti1-Tti2) Complex, the Co-Chaperone of PIKKs and a Potential Target for Cancer Chemotherapy

The heterotrimeric Tel2-Tti1-Tti2 or TTT complex is essential for cell viability and highly observed in eukaryotes. As the co-chaperone of ATR, ATM, DNA-PKcs, mTOR, SMG1, and TRRAP, the phosphatidylinositol 3-kinase-related kinases (PIKKs) and a group of large proteins of 300-500 kDa, the TTT plays crucial roles in genome stability, cell proliferation, telomere maintenance, and aging. Most of the protein kinases in the kinome are targeted by co-chaperone Cdc37 for proper folding and stability. Like Cdc37, accumulating evidence has established the mechanism by which the TTT interacts with chaperone Hsp90 via R2TP (Rvb1-Rvb2-Tah1-Pih1) complex or other proteins for co-translational maturation of the PIKKs. Recent structural studies have revealed the α-solenoid structure of the TTT and its interactions with the R2TP complex, which shed new light on the co-chaperone mechanism and provide new research opportunities. A series of mutations of the TTT have been identified that cause disease syndrome with neurodevelopmental defects, and misregulation of the TTT has been shown to contribute to myeloma, colorectal, and non-small-cell lung cancers. Surprisingly, Tel2 in the TTT complex has recently been found to be a target of ivermectin, an antiparasitic drug that has been used by millions of patients. This discovery provides mechanistic insight into the anti-cancer effect of ivermectin and thus promotes the repurposing of this Nobel-prize-winning medicine for cancer chemotherapy. Here, we briefly review the discovery of the TTT complex, discuss the recent studies, and describe the perspectives for future investigation.

Copper Modulates the Catalytic Activity of Protein Kinase CK2

Casein kinase 2 (CK2) is an evolutionarily conserved serine/threonine kinase implicated in a wide range of cellular functions and known to be dysregulated in various diseases such as cancer. Compared to most other kinases, CK2 exhibits several unusual properties, including dual co-substrate specificity and a high degree of promiscuity with hundreds of substrates described to date. Most paradoxical, however, is its apparent constitutive activity: no definitive mode of catalytic regulation has thus far been identified. Here we demonstrate that copper enhances the enzymatic activity of CK2 both in vitro and in vivo. We show that copper binds directly to CK2, and we identify specific residues in the catalytic subunit of the enzyme that are critical for copper-binding. We further demonstrate that increased levels of intracellular copper result in enhanced CK2 kinase activity, while decreased copper import results in reduced CK2 activity. Taken together, these findings establish CK2 as a copper-regulated kinase and indicate that copper is a key modulator of CK2-dependent signaling pathways.

Protein Kinase CK2 Acts as a Molecular Brake to Control NADPH Oxidase 1 Activation and Colon Inflammation

BACKGROUND & AIMS

NADPH oxidase 1 (NOX1) has emerged as a prime regulator of intestinal mucosa immunity and homeostasis. Dysregulation of NOX1 may cause inflammatory bowel disease (IBD). It is not clear how NOX1 is regulated in vivo under inflammatory conditions. We studied the role of CK2 in this process.

METHODS

The NOX1 organizer subunit, NADPH oxidase organizer 1 (NOXO1), was immunoprecipitated from cytokine-treated colon epithelial cells, and bound proteins were identified by mass spectrometry analysis. Sites on NOXO1 phosphorylated by CK2 were identified by nanoscale liquid chromatography coupled to tandem mass spectrometry. NOX1 activity was determined in colon epithelial cells and colonoids in the presence or absence of CX-4945, a CK2 specific inhibitor. Acute colitis was induced by administration of trinitrobenzenesulfonic acid in mice treated or not with CX-4945. Colon tissues were analyzed by histologic examination, quantitative polymerase chain reaction, and Western blots. CK2 activity, markers of inflammation, and oxidative stress were assessed.

RESULTS

We identified CK2 as a major partner of NOXO1 in colon epithelial cells under inflammatory conditions. CK2 directly binds NOXO1 at the C-terminus containing the Phox homology domain and phosphorylates NOXO1 on several sites. CX-4945 increased ROS generation by NOX1 in human colon epithelial cells and organoids. Strikingly, CK2 activity was reduced in trinitrobenzenesulfonic acid-induced acute colitis, and CX-4945 exacerbated colitis inflammation as shown by increased levels of CXCL1, ROS generation, lipid peroxidation, and colon damage.

CONCLUSIONS

The ubiquitous protein kinase CK2 limits NOX1 activity via NOXO1 binding and phosphorylation in colonic epithelial cells and lessens experimental colitis. Loss of CK2 activity during acute colitis results in excessive ROS production, contributing to the pathogenesis. Strategies to activate CK2 could be an effective novel therapeutic approach in IBD.

CK2 Regulation: Perspectives in 2021

The protein kinase CK2 (CK2) family encompasses a small number of acidophilic serine/threonine kinases that phosphorylate substrates involved in numerous biological processes including apoptosis, cell proliferation, and the DNA damage response. CK2 has also been implicated in many human malignancies and other disorders including Alzheimer′s and Parkinson’s diseases, and COVID-19. Interestingly, no single mechanism describes how CK2 is regulated, including activation by external proteins or domains, phosphorylation, or dimerization. Furthermore, the kinase has an elongated activation loop that locks the kinase into an active conformation, leading CK2 to be labelled a constitutively active kinase. This presents an interesting paradox that remains unanswered: how can a constitutively active kinase regulate biological processes that require careful control? Here, we highlight a selection of studies where CK2 activity is regulated at the substrate level, and discuss them based on the regulatory mechanism. Overall, this review describes numerous biological processes where CK2 activity is regulated, highlighting how a constitutively active kinase can still control numerous cellular activities. It is also evident that more research is required to fully elucidate the mechanisms that regulate CK2 and what causes aberrant CK2 signaling in disease.

How can a traffic light properly work if it is always green? The paradox of CK2 signaling

CK2 is a constitutively active protein kinase that assuring a constant level of phosphorylation to its numerous substrates supports many of the most important biological functions. Nevertheless, its activity has to be controlled and adjusted in order to cope with the varying needs of a cell, and several examples of a fine-tune regulation of its activity have been described. More importantly, aberrant regulation of this enzyme may have pathological consequences, e.g. in cancer, chronic inflammation, neurodegeneration, and viral infection. Our review aims at summarizing our current knowledge about CK2 regulation. In the first part, we have considered the most important stimuli shown to affect protein kinase CK2 activity/expression. In the second part, we focus on the molecular mechanisms by which CK2 can be regulated, discussing controversial aspects and future perspectives.

[Effect of etimizole structural analogues on protein kinase CK2, protein phosphorylation and transcription of chromatin in rat brain cortex and hippocampus]

Protein kinase CK2 is an important enzyme in the nervous system. The nuclear forms of CK2 regulate chromatin structure and gene expression, the key processes for long-term memory formation. Memory modulators, the Structural Analogues of Etimizole (SAE), were able to increase or decrease the activity of chromatin-associated CK in the cortex and hippocampus of rat brain in vitro. In vivo memory enhancers from SAE-group (3 mg/kg) stimulated CK2 activity and the transcriptional ability of chromatin in the cortex and hippocampus, starting from 30 min with a peak for 60 min and a duration up to 180 min. At these periods the memory inhibitor from the SAE-group reduced CK2 activity and chromatin transcription. It is assumed that the modulating effect of SAE on CK2 activity and transcription underlies the effects of these compounds on long-term memory.

Role of phospholipase D in the lifespan of Caenorhabditis elegans

We have previously shown that phospholipase D (PLD) downregulation accelerates cellular senescence, which is widely believed to play an important role in aging, by stimulating reactive oxygen species (ROS) accumulation in human cells. In this study, we examined the role of PLD in aging using the nematode Caenorhabditis elegans. The mRNA level of pld-1 was found to be inversely correlated with aging. RNAi-mediated knockdown of pld-1 expression in nematodes enhanced ROS and lipofuscin accumulation and decreased lifespan, motility, and resistance to stress compared to that in nematodes treated with control RNAi. Pld-1 knockdown repressed the long lifespan of age-1 and akt-1 mutants but did not further reduce the short lifespan of daf-16 mutants, suggesting that PLD functions between AKT-1 and DAF-16. The ROS scavenger N-acetyl-L-cysteine, a PLD effector phosphatidic acid and a possible CK2 activator spermidine attenuated the lifespan shortening and age-related biomarkers triggered by pld-1 knockdown. Pld-1 RNAi downregulated the expression of DAF-16 target genes such as sod-3, dod-11, and mtl-1 in nematodes. In human cells, furthermore, PLD2 downregulation decreased the transcription of FoxO3a target genes (Cu/ZnSOD, MnSOD, catalase, thioredoxin-2, and peroxiredoxin-5), whereas ectopic PLD2 expression elevated the mRNA levels of these antioxidant genes. Taken together, these results indicated that PLD downregulation shortens longevity and induces age-related biomarkers through ROS accumulation by inhibiting the DAF-16/FoxO3a pathway in nematodes.

Modulation of learning and memory by natural polyamines

Spermine and spermidine are natural polyamines that are produced mainly via decarboxylation of l-ornithine and the sequential transfer of aminopropyl groups from S-adenosylmethionine to putrescine by spermidine synthase and spermine synthase. Spermine and spermidine interact with intracellular and extracellular acidic residues of different nature, including nucleic acids, phospholipids, acidic proteins, carboxyl- and sulfate-containing polysaccharides. Therefore, multiple actions have been suggested for these polycations, including modulation of the activity of ionic channels, protein synthesis, protein kinases, and cell proliferation/death, within others. In this review we summarize these neurochemical/neurophysiological/morphological findings, particularly those that have been implicated in the improving and deleterious effects of spermine and spermidine on learning and memory of naïve animals in shock-motivated and nonshock-motivated tasks, from a historical perspective. The interaction with the opioid system, the facilitation and disruption of morphine-induced reward and the effect of polyamines and putative polyamine antagonists on animal models of cognitive diseases, such as Alzheimer's, Huntington, acute neuroinflammation and brain trauma are also reviewed and discussed. The increased production of polyamines in Alzheimer's disease and the biphasic nature of the effects of polyamines on memory and on the NMDA receptor are also considered. In light of the current literature on polyamines, which include the description of an inborn error of the metabolism characterized by mild-to moderate mental retardation and polyamine metabolism alterations in suicide completers, we can anticipate that polyamine targets may be important for the development of novel strategies and approaches for understanding the etiopathogenesis of important central disorders and their pharmacological treatment.

CK2 Secreted by Leishmania braziliensis Mediates Macrophage Association Invasion: A Comparative Study between Virulent and Avirulent Promastigotes

CK2 is a protein kinase distributed in different compartments of Leishmania braziliensis: an externally oriented ecto-CK2, an intracellular CK2, and a secreted CK2. This latter form is constitutively secreted from the parasite (CsCK2), but such secretion may be highly enhanced by the association of specific molecules, including enzyme substrates, which lead to a higher enzymatic activity, called inductively secreted CK2 (IsCK2). Here, we examined the influence of secreted CK2 (sCK2) activity on the infectivity of a virulent L. braziliensis strain. The virulent strain presented 121-fold higher total CK2 activity than those found in an avirulent strain. The use of specific CK2 inhibitors (TBB, DRB, or heparin) inhibited virulent parasite growth, whereas no effect was observed in the avirulent parasites. When these inhibitors were added to the interaction assays between the virulent L. braziliensis strain and macrophages, association index was drastically inhibited. Polyamines enhanced sCK2 activity and increased the association index between parasites and macrophages. Finally, sCK2 and the supernatant of the virulent strain increased the association index between the avirulent strain and macrophages, which was inhibited by TBB. Thus, the kinase enzyme CK2 seems to be important to invasion mechanisms of L. braziliensis.

Spermidine promotes adipogenesis of 3T3-L1 cells by preventing interaction of ANP32 with HuR and PP2A

We have shown previously that the polyamine spermidine is indispensable for differentiation of 3T3-L1 preadipocytes. In the present study, we examined the mechanism of spermidine function by using the polyamine biosynthesis inhibitor α-difluoromethylornithine in combination with the metabolically stable polyamine analogues γ-methylspermidine or (R,R)-α,ω-bismethylspermine. At the early phase of differentiation, spermidine-depleted 3T3-L1 cells showed decreased translation of the transcription factor C/EBPβ (CCAAT/enhancer-binding protein β), decreased PP2A (protein phosphatase 2A) activity and increased cytoplasmic localization of the RNA-binding protein HuR (human antigen R). The amount of HuR bound to C/EBPβ mRNA was reduced, whereas the amount of bound CUGBP2, an inhibitor of C/EBPβ translation, was increased. ANP32 (acidic nuclear phosphoprotein 32) proteins, which are known PP2A inhibitors and HuR ligands, bound more PP2A and HuR in spermidine-depleted than in control cells, whereas immunodepletion of ANP32 proteins from the lysate of spermidine-depleted cells restored PP2A activity. Taken together, our data shows that spermidine promotes C/EBPβ translation in differentiating 3T3-L1 cells, and that this process is controlled by the interaction of ANP32 with HuR and PP2A.

Phosphoproteomic analysis of cells treated with longevity-related autophagy inducers

Macroautophagy is a self-cannibalistic process that enables cells to adapt to various stresses and maintain energy homeostasis. Additionally, autophagy is an important route for turnover of misfolded proteins and damaged organelles, with important implications in cancer, neurodegenerative diseases and aging. Resveratrol and spermidine are able to induce autophagy by affecting deacetylases and acetylases, respectively, and have been found to extend the life-span of model organisms. With the aim to reveal the signaling networks involved in this drug-induced autophagic response, we quantified resveratrol and spermidine-induced changes in the phosphoproteome using SILAC and mass spectrometry. The data were subsequently analyzed using the NetworKIN algorithm to extract key features of the autophagy-responsive kinase-substrate network. We found that two distinct sequence motifs were highly responsive to resveratrol and spermidine and that key proteins modulating the acetylation, phosphorylation, methylation and ubiquitination status were affected by changes in phosphorylation during the autophagic response. Essential parts of the apoptotic signaling network were subjected to post-translational modifications during the drug-induced autophagy response, suggesting potential crosstalk and balancing between autophagy and apoptosis. Additionally, we predicted cellular signaling networks affected by resveratrol and spermidine using a computational framework. Altogether, these results point to a profound crosstalk between distinct networks of post-translational modifications and provide a resource for future analysis of autophagy and cell death.

Radiation protection following nuclear power accidents: a survey of putative mechanisms involved in the radioprotective actions of taurine during and after radiation exposure

There are several animal experiments showing that high doses of ionizing radiation lead to strongly enhanced leakage of taurine from damaged cells into the extracellular fluid, followed by enhanced urinary excretion. This radiation-induced taurine depletion can itself have various harmful effects (as will also be the case when taurine depletion is due to other causes, such as alcohol abuse or cancer therapy with cytotoxic drugs), but taurine supplementation has been shown to have radioprotective effects apparently going beyond what might be expected just as a consequence of correcting the harmful consequences of taurine deficiency per se. The mechanisms accounting for the radioprotective effects of taurine are, however, very incompletely understood. In this article an attempt is made to survey various mechanisms that potentially might be involved as parts of the explanation for the overall beneficial effect of high levels of taurine that has been found in experiments with animals or isolated cells exposed to high doses of ionizing radiation. It is proposed that taurine may have radioprotective effects by a combination of several mechanisms: (1) during the exposure to ionizing radiation by functioning as an antioxidant, but perhaps more because it counteracts the prooxidant catalytic effect of iron rather than functioning as an important scavenger of harmful molecules itself, (2) after the ionizing radiation exposure by helping to reduce the intensity of the post-traumatic inflammatory response, and thus reducing the extent of tissue damage that develops because of severe inflammation rather than as a direct effect of the ionizing radiation per se, (3) by functioning as a growth factor helping to enhance the growth rate of leukocytes and leukocyte progenitor cells and perhaps also of other rapidly proliferating cell types, such as enterocyte progenitor cells, which may be important for immunological recovery and perhaps also for rapid repair of various damaged tissues, especially in the intestines, and (4) by functioning as an antifibrogenic agent. A detailed discussion is given of possible mechanisms involved both in the antioxidant effects of taurine, in its anti-inflammatory effects and in its role as a growth factor for leukocytes and nerve cells, which might be closely related to its role as an osmolyte important for cellular volume regulation because of the close connection between cell volume regulation and the regulation of protein synthesis as well as cellular protein degradation. While taurine supplementation alone would be expected to exert a therapeutic effect far better than negligible in patients that have been exposed to high doses of ionizing radiation, it may on theoretical grounds be expected that much better results may be obtained by using taurine as part of a multifactorial treatment strategy, where it may interact synergistically with several other nutrients, hormones or other drugs for optimizing antioxidant protection and minimizing harmful posttraumatic inflammatory reactions, while using other nutrients to optimize DNA and tissue repair processes, and using a combination of good diet, immunostimulatory hormones and perhaps other nontoxic immunostimulants (such as beta-glucans) for optimizing the recovery of antiviral and antibacterial immune functions. Similar multifactorial treatment strategies may presumably be helpful in several other disease situations (including severe infectious diseases and severe asthma) as well as for treatment of acute intoxications or acute injuries (both mechanical ones and severe burns) where severely enhanced oxidative and/or nitrative stress and/or too much secretion of vasodilatory neuropeptides from C-fibres are important parts of the pathogenetic mechanisms that may lead to the death of the patient. Some case histories (with discussion of some of those mechanisms that may have been responsible for the observed therapeutic outcome) are given for illustration of the likely validity of these concepts and their relevance both for treatment of severe infections and non-infectious inflammatory diseases such as asthma and rheumatoid arthritis.

Specific characteristics of CK2β regulatory subunits in plants

In all eukaryotes, the typical CK2 holoenzyme is an heterotetramer composed of two catalytic (CK2α and CK2α') and two regulatory (CK2β) subunits. One of the distinctive traits of plant CK2 is that they present a greater number of genes encoding for CK2α/β subunits than animals or yeasts, for instance, in Arabidopsis and maize both CK2α/β subunits belong to multigenic families composed by up to four genes. Here, we conducted a genome-wide survey examining 34 different plant genomes in order to investigate if the multigenic property of CK2β genes is a common feature through the entire plant kingdom. Also, at the level of structure, the plant CK2β regulatory subunits present distinctive features as (i) they lack about 20 aminoacids in the C-terminal domain, (ii) they present a specific N-terminal extension of about 90 aminoacids that shares no homology with any previously characterized functional domain, and (iii) the acidic loop region is poorly conserved at the aminoacid level. Since there is no data about CK2β or holoenzyme structure in plants, in this study, we use human CK2β as a template to predict a structure for Zea mays CK2β1 by homology modeling and we discuss about possible structural changes in the acidic loop region that could affect the enzyme regulation.

Protein kinase CK2 links polyamine metabolism to MAPK signalling in Drosophila

MAPK signalling is a complex process not only requiring the core components Raf, MEK and Erk, but also many proteins like the scaffold protein KSR and several kinases to specifically localize, modulate and fine-tune the outcome of the pathway in a cell context specific manner. In mammals, protein kinase CK2 was shown to bind to the scaffold protein KSR and to phosphorylate Raf proteins at a conserved serine residue in the negative-charge regulatory (N-) region, thereby facilitating maximal activity of the MAPK signalling pathway. In this work we show that in Drosophila CK2 is also bound to KSR. However, despite the presence of a corresponding serine residue in the N-region of DRaf, CK2-mediated phosphorylation of DRaf takes place on a serine residue at the N-terminus and is required for Erk activation. Previous work identified polyamines as regulators of CK2 kinase activity. The main cellular source of polyamines is the catabolism of amino acids. Evidence is provided that phosphorylation of DRaf by CK2 is modulated by polyamines, with spermine being the most potent inhibitor of the reaction. We suggest that CK2 is able to monitor intracellular polyamine levels and translates this information to modulate MAPK signalling.

Measurement of polyamine pKa values

The extent of ionization of the polyamines is an important factor in their interactions with cellular components. The pK(a) is the pH at which a functional group is 50% ionized. For compounds such as polyamines with more than one ionizable center (atom or functional group), there is a pK(a) value for each center of ionization. This chapter describes the pK(a) values for each amine group in many important polyamines, the factors influencing these values and methods for their determination using potentiometric titration and nuclear magnetic resonance spectroscopy.

Casein Kinase II: an attractive target for anti-cancer drug design

Casein Kinase II (CK2) is a ubiquitous serine/threonine kinase that is highly conserved in eukaryotic cells. CK2 has been shown to impact cell growth and proliferation, as numerous growth-related proteins are substrates of CK2. More importantly, experimental evidence linking increased expression and activity of CK2 to human cancers underscores the relevance of CK2 biology to cellular transformation and carcinogenesis. Due to the critical regulatory role CK2 plays in cell fate determination in cancer cells, there is a tremendous interest in the development of CK2-specific therapies. Supporting this, recent reports have demonstrated that genetic manipulation of CK2 expression as well as pharmacological inhibition of its enzymatic activity sensitizes cancers to apoptotic stimuli. Here we provide a succinct account of the biology of CK2, its cellular substrates, its pro-survival and pro-proliferation activity, and highlight evidence for its involvement in human cancer.

At the crossroads of homoeostasis and disease: roles of the PACS proteins in membrane traffic and apoptosis

The endomembrane system in mammalian cells has evolved over the past two billion years from a simple endocytic pathway in a single-celled primordial ancestor to complex networks supporting multicellular structures that form metazoan tissue and organ systems. The increased organellar complexity of metazoan cells requires additional trafficking machinery absent in yeast or other unicellular organisms to maintain organ homoeostasis and to process the signals that control proliferation, differentiation or the execution of cell death programmes. The PACS (phosphofurin acidic cluster sorting) proteins are one such family of multifunctional membrane traffic regulators that mediate organ homoeostasis and have important roles in diverse pathologies and disease states. This review summarizes our current knowledge of the PACS proteins, including their structure and regulation in cargo binding, their genetics, their roles in secretory and endocytic pathway traffic, interorganellar communication and how cell-death signals reprogramme the PACS proteins to regulate apoptosis. We also summarize our current understanding of how PACS genes are dysregulated in cancer and how viral pathogens ranging from HIV-1 to herpesviruses have evolved to usurp the PACS sorting machinery to promote virus assembly, viral spread and immunoevasion.

CK2beta gene silencing increases cell susceptibility to influenza A virus infection resulting in accelerated virus entry and higher viral protein content

BACKGROUND

Influenza A virus (IVA) exploits diverse cellular gene products to support its replication in the host. The significance of the regulatory (beta) subunit of casein kinase 2 (CK2beta) in various cellular mechanisms is well established, but less is known about its potential role in IVA replication. We studied the role of CK2beta in IVA-infected A549 human epithelial lung cells.

RESULTS

Activation of CK2beta was observed in A549 cells during virus binding and internalization but appeared to be constrained as replication began. We used small interfering RNAs (siRNAs) targeting CK2beta mRNA to silence CK2beta protein expression in A549 cells without affecting expression of the CK2alpha subunit. CK2beta gene silencing led to increased virus titers, consistent with the inhibition of CK2beta during IVA replication. Notably, virus titers increased significantly when CK2beta siRNA-transfected cells were inoculated at a lower multiplicity of infection. Virus titers also increased in cells treated with a specific CK2 inhibitor but decreased in cells treated with a CK2beta stimulator. CK2beta absence did not impair nuclear export of viral ribonucleoprotein complexes (6 h and 8 h after inoculation) or viral polymerase activity (analyzed in a minigenome system). The enhancement of virus titers by CK2beta siRNA reflects increased cell susceptibility to influenza virus infection resulting in accelerated virus entry and higher viral protein content.

CONCLUSION

This study demonstrates the role of cellular CK2beta protein in the viral biology. Our results are the first to demonstrate a functional link between siRNA-mediated inhibition of the CK2beta protein and regulation of influenza A virus replication in infected cells. Overall, the data suggest that expression and activation of CK2beta inhibits influenza virus replication by regulating the virus entry process and viral protein synthesis.

Molecular mechanism of human Nrf2 activation and degradation: role of sequential phosphorylation by protein kinase CK2

Nrf2 is a key transcription factor in the cellular response to oxidative stress. In this study we identify two phosphorylated forms of endogenous human Nrf2 after chemically induced oxidative stress and provide evidence that protein kinase CK2-mediated sequential phosphorylation plays potential roles in Nrf2 activation and degradation. Human Nrf2 has a predicted molecular mass of 66 kDa. However, immunoblots showed that two bands at 98 and 118 kDa, which are identified as phosphorylated forms, are increased in response to Nrf2 inducers. In addition, human Nrf2 was found to be a substrate for CK2 which mediated two steps of phosphorylation, resulting in two forms of Nrf2 migrating with differing M(r) at 98 kDa (Nrf2-98) and 118 kDa (Nrf2-118). Our results support a role in which calmodulin binding regulates CK2 activity, in that cold (25 degrees C) Ca(2+)-free media (cold/Ca(2+)-free) decreased both cellular calcium levels and CK2-calmodulin binding and induced Nrf2-118 formation, the latter of which was prevented by CK2-specific inhibitors. Gel shift assays showed that the Nrf2-118 generated under cold/Ca(2+)-free conditions does not bind to the antioxidant response element, indicating that Nrf2-98 has transcriptional activity. In contrast, Nrf2-118 is more susceptible to degradation. These results provide evidence for phosphorylation by CK2 as a critical controlling factor in Nrf2-mediated cellular antioxidant response.

Identifying interaction motifs in CK2beta--a ubiquitous kinase regulatory subunit

Casein kinase 2 (CK2) is probably the most ubiquitous serine/threonine kinase found in eukaryotes: it phosphorylates >300 cellular proteins, ranging from transcription factors to proteins involved in chromatin structure and cell division. CK2 is a heterotetrameric enzyme that induces neoplastic growth when overexpressed. The beta subunit of CK2 (CK2beta) functions as the regulator of the catalytic CK2alpha and CK2alpha' subunits, enhancing their stability, activity and specificity. However, CK2beta also functions as a multisubstrate docking platform for several other binding partners. Here, we discuss the organization and roles of interaction motifs of CK2beta, postulate new protein-interaction sites and map these to the known interaction motifs, and show how the resulting complexity of interactions mediated by CK2 gives rise to the versatile functions of this pleiotropic protein kinase.

Protolytic properties of polyamine wasp toxin analogues studied by 13C NMR spectroscopy

Acid-base properties of the natural polyamine wasp toxin PhTX-433 (1) and seven synthetic analogues [PhTX-343 (2), PhTX-334 (3), PhTX-443 (4), PhTX-434 (5), PhTX-344 (6), PhTX-444 (7), and PhTX-333 (8)], each having four protolytic sites, were characterized by 13C NMR spectroscopy. Nonlinear, multiparameter, simultaneous fit of all chemical shift data obtained from the NMR titration curves yielded macroscopic pKa values as well as intrinsic chemical shift data of all differently protonated macrospecies. Analyses of the chemical shift data demonstrated strong interactions between all four sites and provided information about complex relationships between chemical shift values and protonation state. Deprotonation of fully protonated forms starts at the central amino group of the polyamine moiety, and the extent of this trend depends on the distance to the flanking, protonated amino groups. The pKa1 values of 1-8 are in the range 8.2-9.4. Hence, some of the toxins are incompletely protonated at the pH and ionic strength conditions used for assessment of their interactions with ionotropic glutamate and nicotinic acetylcholine receptors, and the degree of protonation is expected to have pharmacological importance in the ion-channel binding event.

Protein kinase CK2 is inhibited by human nucleolar phosphoprotein p140 in an inositol hexakisphosphate-dependent manner

Protein kinase CK2 is a ubiquitous protein kinase that can phosphorylate various proteins involved in central cellular processes, such as signal transduction, cell division, and proliferation. We have shown that the human nucleolar phosphoprotein p140 (hNopp140) is able to regulate the catalytic activity of CK2. Unphosphorylated hNopp140 and phospho-hNopp140 bind to the regulatory and catalytic subunits of CK2, respectively, and the interaction between hNopp140 and CK2 was prevented by inositol hexakisphosphate (InsP(6)). Phosphorylation of alpha-casein, genimin, or human phosphatidylcholine transfer protein-like protein by CK2 was inhibited by hNopp140, and InsP(6) recovered the suppressed activity of CK2 by hNopp140. These observations indicated that hNopp140 serves as a negative regulator of CK2 and that InsP(6) stimulates the activity of CK2 by blocking the interaction between hNopp140 and CK2.

A PACS-1, GGA3 and CK2 complex regulates CI-MPR trafficking

The cation-independent mannose-6-phosphate receptor (CI-MPR) follows a highly regulated sorting itinerary to deliver hydrolases from the trans-Golgi network (TGN) to lysosomes. Cycling of CI-MPR between the TGN and early endosomes is mediated by GGA3, which directs TGN export, and PACS-1, which directs endosome-to-TGN retrieval. Despite executing opposing sorting steps, GGA3 and PACS-1 bind to an overlapping CI-MPR trafficking motif and their sorting activity is controlled by the CK2 phosphorylation of their respective autoregulatory domains. However, how CK2 coordinates these opposing roles is unknown. We report a CK2-activated phosphorylation cascade controlling PACS-1- and GGA3-mediated CI-MPR sorting. PACS-1 links GGA3 to CK2, forming a multimeric complex required for CI-MPR sorting. PACS-1-bound CK2 stimulates GGA3 phosphorylation, releasing GGA3 from CI-MPR and early endosomes. Bound CK2 also phosphorylates PACS-1Ser(278), promoting binding of PACS-1 to CI-MPR to retrieve the receptor to the TGN. Our results identify a CK2-controlled cascade regulating hydrolase trafficking and sorting of itinerant proteins in the TGN/endosomal system.

CK2 interacting proteins: emerging paradigms for CK2 regulation?

Protein kinase CK2 represents a small family of highly conserved protein kinases involved in a complex series of cellular events. Furthermore, CK2 has been localised to many discrete cellular sites and has an extensive and diverse array of substrates and interaction partners in cells. Despite considerable investigation, the precise mechanism(s) of regulation of CK2 in cells remains poorly understood. In consideration of the prospect that cells contain many distinct sub-populations of CK2 that are distinguished on the basis of localisation and/or interactions with other cellular components, one possibility is that there may be differential regulation of specific sub-populations of CK2. With this in mind, some of the individual sub-populations of CK2 may be regulated through particular protein-protein interactions that may play a role in recruiting CK2 into the vicinity of its substrates and/or modulating its ability to phosphorylate specific cellular targets. In this respect, here we examine two CK2-interacting proteins, namely Pin1 and CKIP-1 that have been shown to participate in the modulation of CK2 specificity or the subcellular localisation of CK2, respectively. One aspect of this work has been focused on the prospect that Pin1 interacts with CK2 in response to UV stimulation in a manner analogous to the phosphorylation-dependent interactions of CK2 that occur following the mitotic phosphorylation of CK2. A second aspect of this work involves an examination of the structural basis for interactions between CK2 and CKIP-1 with emphasis on a putative HIKE domain in CK2.

Immunologically defined subclasses of the protein kinase CK2 beta-subunit in prostate carcinoma cell lines

Both, the activity as well as the expression of protein kinase CK2 is enhanced in various cancer types and in established tumour cell lines. This phenomenon is not due to an increase in the CK2 message but rather to posttranscriptional and posttranslational mechanisms. In order to get an insight into these posttranslational modifications we analyzed CK2 in prostate cancer cell lines, which differ by their hormone-sensitivity. We found that the CK2 activity is significantly higher in hormone-refractory than in hormone-sensitive cells although the amount of the catalytic alpha- and alpha'- subunits is comparable. In contrast, we detected seemingly lower amounts of the regulatory beta-subunit in the hormone-refractory cell lines, which later turned out to be an immunologically defined subclass. This subclass is realized by a phosphate group, which is attached to serine 209. The phosphorylation occurs in vivo during mitosis and is executed by the p34(cdc2)/cyclin B kinase. As this phosphorylation enhances the CK2 activity this change might well account for the higher activity of CK2 in prostate cancer cells.

B23 is a downstream target of polyamine-modulated CK2

Our previous studies have shown that the overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, increases the enzymatic activity of the polyamine-responsive enzyme casein kinase 2 (CK2). Because CK2 is known to preferentially associate with the nuclear matrix in response to other trophic stimuli, we investigated the effects of ODC overexpression on CK2 localisation and on the CK2-mediated phosphorylation of a known CK2 substrate, the nucleolar phosphoprotein B23. Immunofluorescence analysis of CK2 and B23 in primary keratinocytes revealed that ODC overexpression resulted in the colocalisation of CK2 with B23 at the nucleolar borders. ODC overexpression also increased CK2 kinase activity 2-fold at the nuclear matrix, a response which could be abrogated by treatment of K6/ODC transgenic keratinocytes with the ODC inhibitor alpha-difluoromethylornithine (DFMO). Levels of B23 protein were also elevated in ODC-overexpressing cells compared to normal cells or transgenic cells treated with DFMO. This increase in protein level was neither due to an increase in steady-state mRNA levels, nor was it due to increased stability of B23 protein. Phosphorylation of B23 was also increased in ODC-overexpressing cells, and this increased phosphorylation could be blocked by treatment of the cells with the CK2 kinase inhibitors apigenin or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). These data suggest that B23 may be a downstream effector of polyamines via phosphorylation by the protein kinase CK2.

Primary and secondary interactions between CK2alpha and CK2beta lead to ring-like structures in the crystals of the CK2 holoenzyme

Protein kinase CK2 predominantly exists as a heterotetrameric holoenyzme consisting of two catalytic subunits (CK2alpha) and two non-catalytic subunits (CK2beta). Early investigations which we review here had revealed the presence of two types of contacts between CK2alpha and CK2beta: a primary interaction responsible for the stability of the CK2 holoenzyme and stimulatory for the catalytic activity, and a secondary interaction which is inhibitory and in which the acidic loop of CK2beta associates with the basic stretch and the (p+1)-loop of CK2alpha. At the end of the last decade both types of interactions were assumed to occur within the same tetrameric complex. The CK2 holoenyzme structure, however, suggested that the secondary interactions must happen between different CK2 tetramers. Such a behaviour should lead to higher-ordered aggregates consistent with several previous reports about a distinct aggregation propensity of CK2. We demonstrate here that in the CK2 holoenzyme crystals contacts between different CK2 tetramers exists which provide structural details of the secondary CK2alpha/CK2beta interactions. These mainly ionic interactions lead to trimeric rings of CK2 holoenzymes in the crystal. In these rings each CK2 tetramer possesses one CK2alpha subunit open for substrate binding and another one whose active site is blocked by a secondary contact with CK2beta from a neighbouring tetramer. This observation fits to previous findings that salt-sensitive ring-like aggregates of CK2 holoenzymes can exist which possess significant catalytic activity. Furthermore it suggests that earlier ideas about a regulatory role of the enzyme's aggregation propensity may be worth to be revitalised.

A Novel Protein Kinase CK2 Substrate Indicates CK2 Is Not Directly Stimulated by Polyamines in Vivo

The activity of the protein kinase (CK2) is enhanced in vitro by the binding of polyamines to the CK2beta regulatory subunit. The overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, also elevates CK2 kinase activity in primary keratinocytes and tissues of K6/ODC transgenic mice. In an effort to better characterize the mechanisms by which polyamines may affect CK2 in vivo, we constructed a transfectable CK2 substrate cDNA consisting of the enhanced green fluorescence protein appended with a canonical CK2 phosphorylation sequence (EGFP-S). In contrast to unmodified EGFP, the EGFP-S protein was extensively phosphorylated by CK2, and this phosphorylation was stimulated by the polyamine spermine in a dose-dependent manner. The in vivo phosphorylation of EGFP-S was examined in cell lines which inducibly express either wild-type CK2 holoenzyme or a CK2 holoenzyme which contains activating mutations in the polyamine-binding region of its CK2beta regulatory subunit. Neither the overexpression of ODC in either cell line nor the mutation of the CK2beta subunit conferred an increase in CK2 kinase activity as measured by the in vivo phosphorylation of EGFP-S. Rather, our data indicate that polyamines increase total CK2 kinase activity through increases in steady-state levels of both CK2alpha and CK2beta subunits. The overexpression of ODC resulted in a 3-fold increase in steady-state levels of both exogenous and endogenous CK2 transcripts but did not increase the half-life of wild-type or mutated CK2 protein. These data suggest that the regulation of intracellular CK2 by the polyamines may occur through mechanisms distinct from the direct stimulation of CK2 by polyamines in vitro as previously described.

Depletion of polyamines and increase of transforming growth factor-beta1, c-myc, collagen-type I, matrix metalloproteinase-1, and metalloproteinase-2 mRNA in primary human gingival fibroblasts

BACKGROUND

The polyamines spermidine, spermine, and putrescine are known to be deeply linked with growth processes, gene expression, and extracellular matrix synthesis. Their cellular content depends primarily on the activity of the enzyme ornithine decarboxylase. High levels of ornithine decarboxylase and polyamines have been found in proliferative, inflammatory, and neoplastic pathologies of the oral cavity and in gingival fluid. Difluoromethylornithine (DFMO) selectively inhibits ornithine decarboxylase, thus depleting polyamine content and preventing cell proliferation and synthesis activity. The aim of this study was to investigate whether DFMO treatment could modify the genes involved in cell proliferation and extracellular matrix turnover.

METHODS

Fibroblasts derived from non-inflamed gingiva were maintained in Dulbecco's modified Eagle's medium (DMEM) plus alpha-difluoromethylornithine for 4 days. At 0, 24, 48, 72, and 96 hours cell number was assessed, polyamine levels were quantified with high performance liquid chromatography (HPLC) method, and transforming growth factor-beta1 (TGF-beta1), c-myc, matrix metalloproteinases (MMP)-1 and 2, collagen type I (COL-I) and tissue inhibitor of matrix metalloproteinases (TIMP)-1 were evaluated by reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

Fibroblasts treated with DFMO significantly decreased cell proliferation, ornithine decarboxylase activity, and putrescine levels at all treatment times, spermidine after 72 and 96 hours, and spermine after 96 hours of culture. Total polyamines decreased (P < or =0.01) at 96 hours after DFMO treatment, while c-myc, TGF-beta1, MMP-1 and 2, COL-I mRNA significantly increased. Conversely, TIMP-1 did not show any significant change. The polyamines trend was not correlated to c-myc, TGF-beta1, MMP-1 and -2, and TIMP-1 mRNA levels. Transforming growth factor-beta1 and c-myc mRNA expression were related and correlated to MMP-1 and 2, COL-I and TIMP-1 mRNA trend after DFMO treatment.

CONCLUSIONS

Our data show that as the polyamine content decreases, TGF-beta1, c-myc, MMP-1 and -2, and COL-I mRNA levels increase, therefore a negative regulatory role of the polyamines on the mRNA expression could be suggested.

Order or chaos? An evaluation of the regulation of protein kinase CK2

CK2 is a highly conserved, ubiquitously expressed protein serine/threonine kinase present in all eukaryotes. Circumscribed as having a vast array of substrates located in a number of cellular compartments, CK2 has been implicated in critical cellular processes such as proliferation, apoptosis, differentiation, and transformation. Despite advances in elucidating its substrates and involvement in cellular regulation, its precise mode of regulation remains poorly defined. In this respect, there are currently conflicting views as to whether CK2 is constitutively active or modulated in response to specific stimuli. Perhaps an important consideration in resolving these apparent discrepancies is recognition of the existence of many discrete CK2 subpopulations that are distinguished from one another by localization or association with distinct cellular components. The existence of these subpopulations brings to light the possibility of each population being regulated independently rather than the entire cellular CK2 content being regulated globally. Logically, each local population may then be regulated in a distinct manner to carry out its precise function(s). This review will examine those mechanisms including regulated expression and assembly of CK2 subunits, phosphorylation of CK2, and interactions with small molecules or cellular proteins that could contribute to the local regulation of distinct CK2 populations.

Molecular mechanisms of polyamine analogs in cancer cells

The natural polyamines are aliphatic cations with multiple functions and are essential for cell growth. Soon after the critical requirement of polyamines for cell proliferation was recognized, the metabolism of polyamines was pursued as a target for antineoplastic therapy. Initially, much attention was focused on the development of inhibitors of polyamine biosynthesis as a means to inhibit tumor growth. The best-characterized inhibitor is alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase. While compensatory mechanisms in polyamine metabolism reduce the effectiveness of DFMO as a single chemotherapeutic agent, it is currently undergoing extensive testing and clinical trials for chemoprevention and other diseases. There has been increasing interest over the last two decades in the cytotoxic response to agents that target the regulation of polyamine metabolism rather than directly inhibiting the metabolic enzymes in tumor cells. This interest resulted in the development of a number of polyamine analogs that exhibit effective cytotoxicity against tumor growth in preclinical models. The analogs enter cells through a selective polyamine transport system and can be either polyamine antimetabolites that deplete the intracellular polyamines or polyamine mimetics that displace the natural polyamines from binding sites, but do not substitute in terms of growth-promoting function. Synthesis of the first generation of symmetrically substituted bis(alkyl)polyamine analogs in the mid-1980s was based on the theory that polyamines may utilize feedback mechanisms to auto-regulate their synthesis. In the 1990s, unsymmetrically substituted bis(alkyl) polyamine analogs were developed. These compounds display structure-dependent and cell type-specific cellular effects and regulation on polyamine metabolism. More recently, a novel class of analogs has been synthesized, which include conformationally restricted, cyclic and long-chain oligoamine analogs. The development and use of these analogs have provided valuable information for understanding the molecular mechanisms of targeting the polyamine pathway as a means of cancer therapy.

Regulation of apoptosis signal-regulating kinase 1 (ASK1) by polyamine levels via protein phosphatase 5

Recent evidence has implicated the protein phosphatase PP5 in a variety of signaling pathways. Whereas several proteins have been identified that interact with PP5 and regulate its activity, a possibility of its regulation by second messengers remains speculative. Activation of PP5 in vitro by polyunsaturated fatty acids (e.g. arachidonic acid) and fatty acyl-CoA esters (e.g. arachidonoyl-CoA) has been reported. We report here that PP5 is strongly inhibited by micromolar concentrations of a natural polyamine spermine. This inhibition was observed both in assays with a low molecular weight substrate p-nitrophenyl phosphate as well as phosphocasein and apoptosis signal-regulating kinase 1 (ASK1), thought to be a physiological substrate of PP5. Furthermore, a decrease in polyamine levels in COS-7 cells induced by alpha-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, led to accelerated dephosphorylation of oxidative stress-activated ASK1. This effect was suppressed by okadaic acid and by siRNA-mediated PP5 depletion, indicating that the effect of polyamine levels on ASK1 dephosphorylation was mediated by PP5. In line with the decreased ASK1 activation, polyamine depletion in COS-7 cells abrogated oxidative stress-induced activation of caspase-3, which executes ASK1-induced apoptosis, as well as caspase-3 activation induced by ASK1 overexpression, but had no effect on basal caspase-3 activity. These results implicate polyamines, emerging intracellular signaling molecules, as potential physiological regulators of PP5. Our findings also suggest a novel mechanism of the anti-apoptotic action of a decrease in polyamine levels via de-inhibition of PP5 and accelerated dephosphorylation and deactivation of ASK1.

The multiple personalities of the regulatory subunit of protein kinase CK2: CK2 dependent and CK2 independent roles reveal a secret identity for CK2beta

Protein kinase CK2 (formerly casein kinase II), an enzyme that participates in a wide variety of cellular processes, has traditionally been classified as a stable tetrameric complex consisting of two catalytic CK2α or CK2α' subunits and two regulatory CK2β subunits. While consideration of CK2 as a tetrameric complex remains relevant, significant evidence has emerged to challenge the view that its individual subunits exist exclusively within these complexes. This review will summarize biochemical and genetic evidence indicating that the regulatory CK2β subunit exists and performs functions independently of CK2 tetramers. For example, unbalanced expression of catalytic and regulatory CK2 subunits has been observed in a variety of tissues and tumors. Furthermore, localization studies including live cell imaging have demonstrated that while the catalytic and regulatory subunits of CK2 exhibit extensive co-localization, independent mobility of the individual CK2 subunits can also be observed within cells. Identification of proteins that interact with CK2β in the absence of catalytic CK2 subunits reinforces the notion that CK2β has functions distinct from CK2 and begins to offer insights into these CK2-independent functions. In this respect, the discovery that CK2β can interact with and modulate the activity of a number of other serine/threonine protein kinases including A-Raf, c-Mos and Chk1 is particularly striking. This review will discuss the interactions between CK2β and these protein kinases with special emphasis on the properties of CK2β that mediate these interactions and on the implications of these interactions in yielding new prospects for elucidation of the cellular functions of CK2β.

Ornithine decarboxylase, polyamines and CD11b expression in HL-60 cells during differentiation induced by retinoic acid

Polyamines (PA) and retinoic acid affect mammalian cell growth, differentiation and apoptosis. Retinoic acid induces granulocytic differentiation of mieloid cell lines and, during this process, is responsible for the expression of CD11b, a surface antigen. In this study we investigate the effects of retinoic acid on HL-60 cells, monitoring ornithine decarboxylase (ODC) activity (enzyme rate of PA), putrescine (PUT), spermidine (SPD), spermine (SPM) levels, CD11b myeloid surface marker differentiation, cell cycle, and apoptosis. ODC activity and PUT levels are correlated with mieloid cell differentiation induced by retinoic acid treatment. Only the ODC/PUT ratio is connected with retinoic acid treated HL-60 cells. Treated cultures show a decrease of proliferation and a cell block in the G0/G1 phase, with consequent diminished S phase. The G0/G1 and S phases are significantly related to ODC activity and to PUT and SPD behavior, whereas in differentiating condition only the decrease of PUT is related to the S phase. CD11b expression, stimulated by retinoic acid treatment, is associated with the SPM trend. Total PA behavior agrees with apoptotic cell increase after 96 h of stimulation. Our data show that retinoic acid treatment modifies ODC activity and the turnover of PA. PUT, SPD and SPM, therefore, have a different role, and may be involved in the differentiative/apoptotic program of retinoic acid treated HL-60 cells.

Determination of a large number of kinase activities using peptide substrates, P81 phosphocellulose paper arrays and phosphor imaging

To perform phosphoproteomics and signal transduction studies, a number of protein kinase activities and levels must be simultaneously analyzed in different cell samples and correlated with phosphoprotein patterns to obtain conclusions with regard to the regulation of kinase networks. We describe here a miniaturized format of the classical phosphocellulose (P81) paper binding assay with which up to 594 kinase reactions can be simultaneously analyzed. Kinase peptide substrates possessing a minimum of three consecutive basic residues were subjected to phosphorylation in 96-well plates and aliquots of the phosphorylation reactions were spotted on arrays printed on P81 papers. Phosphorylation levels were quantified using a storage phosphor system imager. The versatility of the procedure was validated by analyzing casein kinase 2, protein kinase C, and p34cdc2/cyclin B in cell extracts and testing the effect of known inhibitors and activators on kinase activities. This improved, miniaturized version of the classical P81 paper method combines simplicity, high sensitivity, high reproducibility, high reliability, and optimal Z factors and takes into account possible sources of background signals. We discuss the possibility of automation and the advantages over other methods.

The protein kinase 60S is a free catalytic CK2alpha' subunit and forms an inactive complex with superoxide dismutase SOD1

The 60S ribosomes from Saccharomyces cerevisiae contain a set of acidic P-proteins playing an important role in the ribosome function. Reversible phosphorylation of those proteins is a mechanism regulating translational activity of ribosomes. The key role in regulation of this process is played by specific, second messenger-independent protein kinases. The PK60S kinase was one of the enzymes phosphorylating P-proteins. The enzyme has been purified from yeast and characterised. Pure enzyme has properties similar to those reported for casein kinase type 2. Peptide mass fingerprinting (PMF) has identified the PK60S as a catalytic alpha(') subunit of casein kinase type 2 (CK2alpha(')). Protein kinase activity is inhibited by SOD1 and by highly specific CK2 inhibitor-4,5,6,7-tetrabromo-benzotriazole (TBBt). The possible mechanism of regulation of CK2alpha(') activity in stress conditions, by superoxide dismutase in regulation of 80S-ribosome activity, is discussed.

CK2 phosphorylation of the armadillo repeat region of beta-catenin potentiates Wnt signaling

Protein kinase CK2 is a ubiquitous serine/threonine kinase involved in many biological processes. It is overexpressed in many malignancies including rodent and human breast cancer, and is up-regulated in Wnt-transfected mammary epithelial cells, where it can be found in a complex with dishevelled and beta-catenin. beta-Catenin is a substrate for CK2 and inhibition of CK2 reduces levels of beta-catenin and dishevelled. Here we report that inhibition of CK2 using pharmacologic agents or expression of kinase inactive subunits reduces beta-catenin-dependent transcription and protein levels in a proteasome-dependent fashion. The major region of phosphorylation of beta-catenin by CK2 is the central armadillo repeat domain, where carrier proteins like axin and the adenomatous polyposis coli gene product APC interact with beta-catenin. The major CK2 phosphorylation site in this domain is Thr393, a solvent-accessible residue in a key hinge region of the molecule. Mutation of this single amino acid reduces beta-catenin phosphorylation, cotranscriptional activity, and stability. Thus, CK2 is a positive regulator of Wnt signaling through phosphorylation of beta-catenin at Thr393, leading to proteasome resistance and increased protein and co-transcriptional activity.

Structural basis of regulation and substrate specificity of protein kinase CK2 deduced from the modeling of protein-protein interactions

BACKGROUND

Protein Kinase Casein Kinase 2 (PKCK2) is an ubiquitous Ser/Thr kinase expressed in all eukaryotes. It phosphorylates a number of proteins involved in various cellular processes. PKCK2 holoenzyme is catalytically active tetramer, composed of two homologous or identical and constitutively active catalytic (alpha) and two identical regulatory (beta) subunits. The tetramer cannot phosphorylate some substrates that can be phosphorylated by PKCK2alpha in isolation. The present work explores the structural basis of this feature using computational analysis and modeling.

RESULTS

We have initially built a model of PKCK2alpha bound to a substrate peptide with a conformation identical to that of the substrates in the available crystal structures of other kinases complexed with the substrates/ pseudosubstrates. In this model however, the fourth acidic residue in the consensus pattern of the substrate, S/T-X-X-D/E where S/T is the phosphorylation site, did not result in interaction with the active form of PKCK2alpha and is highly solvent exposed. Interaction of the acidic residue is observed if the substrate peptide adopts conformations as seen in beta turn, alpha helix, or 3(10) helices. This type of conformation is observed and accommodated well by PKCK2alpha in calmodulin where the phosphorylation site is at the central helix. PP2A carries sequence patterns for PKCK2alpha phosphorylation. While the possibility of PP2A being phosphorylated by PKCK2 has been raised in the literature we use the model of PP2A to generate a model of PP2A-PKCK2alpha complex. PKCK2beta undergoes phosphorylation by holoenzyme at the N-terminal region, and is accommodated very well in the limited space available at the substrate-binding site of the holoenzyme while the space is insufficient to accommodate the binding of PP2A or calmodulin in the holoenzyme.

CONCLUSION

Charge and shape complimentarity seems to play a role in substrate recognition and binding to PKCK2alpha, along with the consensus pattern. The detailed conformation of the substrate peptide binding to PKCK2 differs from the conformation of the substrate/pseudo substrate peptide that is bound to other kinases in the crystal structures reported. The ability of holoenzyme to phosphorylate substrate proteins seems to depend on the accessibility of the P-site in limited space available in holoenzyme.

Molecular features of adult mouse small intestinal epithelial progenitors

The adult mouse small intestinal epithelium undergoes perpetual regeneration, fueled by a population of multipotential stem cells and oligopotential daughters located at the base of crypts of Lieberkühn. Although the morphologic features of small intestinal epithelial progenitors (SiEPs) are known, their molecular features are poorly defined. Previous impediments to purification and molecular characterization of SiEPs include lack of ex vivo clonigenic assays and the difficulty of physically retrieving them from their niche where they are interspersed between their numerous differentiated Paneth cell daughters. To overcome these obstacles, we used germ-free transgenic mice lacking Paneth cells to obtain a consolidated population of SiEPs with normal proliferative activity. These cells were harvested by laser capture microdissection. Functional genomics analysis identified 163 transcripts enriched in SiEPs compared with Paneth cell-dominated normal crypt base epithelium. The dataset was validated by (i) correlation with the organellar composition of SiEPs versus Paneth cells, (ii) similarities to databases generated from recent mouse hematopoietic and neural stem cell genome anatomy projects, and (iii) laser capture microdissectionreal-time quantitative RT-PCR studies of progenitor cell-containing populations retrieved from the small intestines, colons, and stomachs of conventionally raised mice. The SiEP profile has prominent representation of genes involved in c-myc signaling and in the processing, localization, and translation of mRNAs. This dataset, together with our recent analysis of gene expression in the gastric stem cell niche, discloses a set of molecular features shared by adult mouse gut epithelial progenitors.

Live-cell fluorescence imaging reveals the dynamics of protein kinase CK2 individual subunits

Protein kinase CK2 is a multifunctional enzyme which has long been described as a stable heterotetrameric complex resulting from the association of two catalytic (alpha or alpha') and two regulatory (beta) subunits. To track the spatiotemporal dynamics of CK2 in living cells, we fused its catalytic alpha and regulatory beta subunits with green fluorescent protein (GFP). Both CK2 subunits contain nuclear localization domains that target them independently to the nucleus. Imaging of stable cell lines expressing low levels of GFP-CK2alpha or GFP-CK2beta revealed the existence of CK2 subunit subpopulations exhibiting differential dynamics. Once in the nucleus, they diffuse randomly at different rates. Unlike CK2beta, CK2alpha can shuttle, showing the dynamic nature of the nucleocytoplasmic trafficking of the kinase. When microinjected in the cytoplasm, the isolated CK2 subunits are rapidly translocated into the nucleus, whereas the holoenzyme complex remains in this cell compartment, suggesting an intramolecular masking of the nuclear localization sequences that suppresses nuclear accumulation. However, binding of FGF-2 to the holoenzyme triggers its nuclear translocation. Since the substrate specificity of CK2alpha is dramatically changed by its association with CK2beta, the control of the nucleocytoplasmic distribution of each subunit may represent a unique potential regulatory mechanism for CK2 activity.

Protein kinase CK2: structure, regulation and role in cellular decisions of life and death

Protein kinase CK2 ('casein kinase II') has traditionally been classified as a messenger-independent protein serine/threonine kinase that is typically found in tetrameric complexes consisting of two catalytic (alpha and/or alpha') subunits and two regulatory beta subunits. Accumulated biochemical and genetic evidence indicates that CK2 has a vast array of candidate physiological targets and participates in a complex series of cellular functions, including the maintenance of cell viability. This review summarizes current knowledge of the structural and enzymic features of CK2, and discusses advances that challenge traditional views of this enzyme. For example, the recent demonstrations that individual CK2 subunits exist outside tetrameric complexes and that CK2 displays dual-specificity kinase activity raises new prospects for the precise elucidation of its regulation and cellular functions. This review also discusses a number of the mechanisms that contribute to the regulation of CK2 in cells, and will highlight emerging insights into the role of CK2 in cellular decisions of life and death. In this latter respect, recent evidence suggests that CK2 can exert an anti-apoptotic role by protecting regulatory proteins from caspase-mediated degradation. The mechanistic basis of the observation that CK2 is essential for viability may reside in part in this ability to protect cellular proteins from caspase action. Furthermore, this anti-apoptotic function of CK2 may contribute to its ability to participate in transformation and tumorigenesis.