Selectivity of 4,5,6,7-tetrabromobenzotriazole, an ATP site-directed inhibitor of protein kinase CK2 ('casein kinase-2')

Protein kinase CK2 catalyzes tyrosine phosphorylation in mammalian cells

Protein kinase CK2 exhibits oncogenic activity in mice and is over-expressed in a number of tumors or leukemic cells. On the basis of its amino acid sequence and a wealth of experimental information, CK2 has traditionally been classified as a protein serine/threonine kinase. In contrast to this traditional view of CK2, recent evidence has shown that CK2 can also phosphorylate tyrosine residues under some circumstances in vitro and in yeast. In this study, we provide definitive evidence demonstrating that CK2 also exhibits tyrosine kinase activity in mammalian cells. Tyrosine phosphorylation of CK2 in cells and in CK2 immunoprecipitates is dependent on CK2 activity and is inhibited by the CK2 selective inhibitor 4,5,6,7-tetrabromobenzotriazole. Examination of phosphotyrosine profiles in cells reveals a number of proteins, including CK2 itself, which exhibit increased tyrosine phosphorylation when CK2 levels are increased. Peptide arrays to evaluate the specificity determinants for tyrosine phosphorylation by CK2 reveal that its specificity for tyrosine phosphorylation is distinct from its specificity for serine/threonine phosphorylation. Of particular note is the requirement for an aspartic acid immediately C-terminal to the phosphorylatable tyrosine residue. Collectively, these data provide conclusive evidence that CK2 catalyzes the phosphorylation of tyrosine residues in mammalian cells, a finding that adds a new level of complexity to the challenge of elucidating its cellular functions. Furthermore, these results raise the possibility that increased CK2 levels that frequently accompany transformation may contribute to the increased tyrosine phosphorylation that occurs in transformed cells.

A role for CK2 upon interkinetic nuclear migration in the cell cycle of retinal progenitor cells

In developing retina, the nucleus of the elongated neuroepithelial cells undergoes interkinetic nuclear migration (INM), that is it migrates back and forth across the proliferative layer during the cell cycle. S-phase occurs at the basal side, while M-phase occurs at the apical margin of the retinal progenitors. G1 and G2-phases occur along the nuclear migration pathway. We tested whether this feature of the retinal cell cycle is controlled by CK2, which, among its many substrates, phosphorylates both molecular motors and cytoskeletal components. Double immunolabeling showed that CK2 is contained in BrdU-labeled retinal progenitors. INM was examined after pulse labeling the retina of newborn rats with BrdU, by plotting nuclear movement from basal to apical sides of the retinal progenitors during G2. The CK2 specific inhibitor 4,5,6,7-tetrabromobenzotriazole inhibited the activity of rat retinal CK2, and blocked nuclear movement proper in a dose-dependent way. No apoptosis was detected, and total numbers of BrdU-labeled nuclei remained constant following treatment. Immunohistochemistry showed that, following inhibition of CK2, the tubulin cytoskeleton is disorganized, with reduced acetylated and increased tyrosinated tubulin. This indicates a reduction in stable microtubules, with accumulation of free tubulin dimers. The results show that CK2 activity is required for INM in retinal progenitor cells.

A phosphatase cascade by which rewarding stimuli control nucleosomal response

Dopamine orchestrates motor behaviour and reward-driven learning. Perturbations of dopamine signalling have been implicated in several neurological and psychiatric disorders, and in drug addiction. The actions of dopamine are mediated in part by the regulation of gene expression in the striatum, through mechanisms that are not fully understood. Here we show that drugs of abuse, as well as food reinforcement learning, promote the nuclear accumulation of 32-kDa dopamine-regulated and cyclic-AMP-regulated phosphoprotein (DARPP-32). This accumulation is mediated through a signalling cascade involving dopamine D1 receptors, cAMP-dependent activation of protein phosphatase-2A, dephosphorylation of DARPP-32 at Ser 97 and inhibition of its nuclear export. The nuclear accumulation of DARPP-32, a potent inhibitor of protein phosphatase-1, increases the phosphorylation of histone H3, an important component of nucleosomal response. Mutation of Ser 97 profoundly alters behavioural effects of drugs of abuse and decreases motivation for food, underlining the functional importance of this signalling cascade.

Protein kinase CK2 as a druggable target

CK2 is probably the most pleiotropic Ser/Thr protein kinase with hundreds of endogenous substrates already known, which are implicated in a variety of cellular functions. At variance with most protein kinases whose activity is turned on only in response to specific stimuli, and whose genetic alterations often underlie pathological situations, CK2 is not susceptible to tight regulation and there are no mutations known to affect its constitutive activity. Nevertheless an abnormally high level of CK2 is invariably found in tumours, and solid arguments have accumulated suggesting that CK2 plays a global pro-survival function, which under special circumstances creates a cellular environment particularly favourable to the development and potentiation of the tumour phenotype. Therefore any strategy aimed at attenuating CK2 activity may represent a "master key" for the treatment of different neoplastic diseases. Waiting for the clarification of the epigenetic mechanisms promoting the rise of CK2 in cells predisposed to develop a tumour phenotype, a useful pharmacological aid can come from the improvement of a number of fairly potent and selective CK2 inhibitors already available.

Modulation of extracellular matrix protein phosphorylation alters mineralization in differentiating chick limb-bud mesenchymal cell micromass cultures

Protein phosphorylation and dephosphorylation are important regulators of cellular and extracellular events. The purpose of this study was to define how these events regulate cartilage matrix calcification in a cell culture system that mimics endochondral ossification. The presence of casein kinase II (CK2), an enzyme known to phosphorylate matrix proteins, was confirmed by immunohistochemistry. The importance of phosphoprotein phosphorylation and dephosphorylation was examined by comparing effects of inhibiting CK2 or phosphoprotein phosphatases on mineral accretion relative to untreated mineralizing controls. Specific inhibitors were added to differentiating chick limb-bud mesenchymal cell micromass cultures during the development of a mineralized matrix at the times of cell differentiation, proliferation, formation of the mineralized matrix, or proliferation of the mineral crystals. The mineralizing media for these cultures contained 4 mM inorganic phosphate and no organic-phosphate esters; control cultures had 1 mM inorganic phosphate. Mineralization was monitored based on (45)Ca uptake and infrared characterization of the mineral; cell viability was assessed by three independent methods. Treatments that caused cell toxicity were excluded from the analysis. Inhibition of CK2 activity with apigenin or CK2 inhibitor II reduced the rate of mineral deposition, but did not block mineral accretion. Effects were greatest during the time of mineralized matrix formation. Inhibition of phosphoprotein phosphatase activities with okadaic acid, calyculin A, and microcystin-LR, at early time points also markedly inhibited mineral accretion. Inhibition after mineralization had commenced increased the mineral yield. Levamisole, an alkaline phosphatase inhibitor, had no effect on mineral accretion in this system, suggesting the involvement of other phosphatases. Adding additional inorganic phosphate to the inhibited cultures after mineralization had started, but not earlier, reversed the inhibition indicating that the phosphatases were, in part, providing a source of inorganic phosphate. To characterize the roles of specific phosphoproteins blocking studies were performed. Blocking with anti-osteopontin antibody confirmed osteopontin's previously reported role as a mineralization inhibitor. Blocking antibodies to bone sialoprotein added from day 9 or on days 9 and 11 retarded mineralization, supporting its role as a mineralization nucleator. Antibodies to osteonectin slightly stimulated early mineralization, but had no effect after the time that initial mineral deposition occurs. Taken together, the results of this study demonstrate the importance of the phosphorylation state of extracellular matrix proteins in regulating mineralization in this culture system.

Role of heme oxygenase-2 in pial arteriolar response to acetylcholine in mice with and without transfusion of cell-free hemoglobin polymers

Carbon monoxide derived from heme oxygenase (HO) may participate in cerebrovascular regulation under specific circumstances. Previous work has shown that HO contributes to feline pial arteriolar dilation to acetylcholine after transfusion of a cell-free polymeric hemoglobin oxygen carrier. The role of constitutive HO2 in the pial arteriolar dilatory response to acetylcholine was determined by using 1) HO2-null mice (HO2-/-), 2) the HO inhibitor tin protoporphyrin IX (SnPPIX), and 3) 4,5,6,7-tetrabromobenzotriazole (TBB), an inhibitor of casein kinase-2 (CK2)-dependent phosphorylation of HO2. In anesthetized mice, superfusion of a cranial window with SnPPIX decreased arteriolar dilation produced by 10 microM acetylcholine by 51%. After partial polymeric hemoglobin exchange transfusion, the acetylcholine response was normal but was reduced 72% by SnPPIX and 95% by TBB. In HO2-/- mice, the acetylcholine response was modestly reduced by 14% compared with control mice and was unaffected by SnPPIX. After hemoglobin transfusion in HO2-/- mice, acetylcholine responses were also unaffected by SnPPIX and TBB. In contrast, nitric oxide synthase inhibition completely blocked the acetylcholine responses in hemoglobin-transfused HO2-/- mice. We conclude 1) that HO2 activity partially contributes to acetylcholine-induced pial arteriolar dilation in mice, 2) that this contribution is augmented in the presence of a plasma-based hemoglobin polymer and appears to depend on a CK2 kinase mechanism, 3) that nitric oxide synthase activity rather than HO1 activity contributes to the acetylcholine reactivity in HO2-/- mice, and 4) that plasma-based polymeric hemoglobin does not scavenge all of the nitric oxide generated by cerebrovascular acetylcholine stimulation.

Protein kinase CK2 modulates synaptic plasticity by modification of synaptic NMDA receptors in the hippocampus

Synaptic plasticity is the foundation of learning and memory. The protein kinase CK2 phosphorylates many proteins related to synaptic plasticity, but whether it is directly involved in it has not been clarified. Here, we examined the role of CK2 in synaptic plasticity in hippocampal slices using the CK2 selective inhibitors 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) and 4,5,6,7-tetrabromobenzotriazole (TBB). These significantly inhibited N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP). DRB also inhibited NMDA receptor-mediated synaptic transmission, while leaving NMDA receptor-independent LTP unaffected. NMDA receptors thus appear to be the primary targets of CK2. Although both long-term depression (LTD) and LTP are induced by the influx of Ca(2+) through NMDA receptors, surprisingly, LTD was not affected by CK2 inhibitors. We postulated that the LTP-selective modulation by CK2 is due to selective modulation of NMDA receptors, and tested two hypotheses concerning the modulation of NMDA receptors: (i) CK2 selectively modulates NR2A subunits possibly related to LTP, but not NR2B subunits possibly related to LTD; and (ii) CK2 selectively affects synaptic but not extrasynaptic NMDA receptors whose activation is sufficient to induce LTD. DRB decreased NMDA receptor-mediated synaptic transmission in the presence of selective NR2A subunit antagonist. The former hypothesis thus appears unlikely to be correct. However, DRB decreased synaptic NMDA receptor responses in cultured hippocampal neurons without affecting extrasynaptic NMDA receptor current. These findings support the latter hypothesis, that CK2 selectively affects LTP by selective modification of synaptic NMDA receptors in a receptor-location-specific manner.

Inhibition of casein kinase 2 enhances the death ligand- and natural kiler cell-induced hepatocellular carcinoma cell death

Recent studies have shown that the inhibition of casein kinase 2 (CK2) sensitizes many cancer cells to Fas ligand- and tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. However, it has not been demonstrated directly whether CK2 inhibition can also enhance the cytotoxicity of natural killer (NK) cells, which actually use the death ligands to kill cancer cells in vivo. To address whether NK cell-mediated cancer cell death is affected by the inhibition of CK2, we first checked whether the death ligand-induced apoptosis of hepatocellular carcinoma cells (HCCs) and HeLa were affected by CK2 inhibition. We then investigated the effect of CK2 inhibition on NK cytotoxicity against HCCs and HeLa cells and its mechanistic features. Inhibition of CK2 by emodin increased the apoptotic cell death of HepG2, Hep3B and HeLa when the cancer cell lines were treated with a soluble form of recombinant TRAIL or an agonistic antibody of Fas. This phenomenon appeared to be correlated with the expression level of death receptors on the cancer cell surface. More interestingly, the inhibition of CK2 also greatly increased the NK cell-mediated cancer cell killing. The NK cytotoxicity against the cancer cells increased about twofold when the target cells were pretreated with a specific CK2 inhibitor, emodin or 4,5,6,7-tetrabromobenzotriazole. Furthermore, the increase of the NK cytotoxicity against cancer cells by CK2 inhibition was granule-independent and mediated possibly by the death ligands on the NK cell surface. This suggests that CK2 inhibitors could be used to enhance the cytotoxicity of NK cells and consequently increase host tumour immunity.

High throughput screening reveals several new classes of glutamate dehydrogenase inhibitors

Glutamate dehydrogenase (GDH) has been shown to play a regulatory role in insulin secretion by pancreatic beta-cells. The most compelling evidence of this comes from features of the hyperinsulism/hyperammonemia (HI/HA) syndrome where a dominant mutation causes the loss of inhibition by GTP, and from studies that link leucine (and its analogue BCH) activation of GDH to stimulation of insulin secretion. This suggests that GDH may represent a new and novel drug target to control a variety of insulin disorders. Recently we demonstrated that a subset of green tea polyphenols are potent inhibitors of glutamate dehydrogenase in vitro and can efficaciously block BCH stimulation of insulin secretion. In these current studies, we extend our search for GDH inhibitors using high throughput methods to pan through more than 27,000 compounds. A number of known and new inhibitors were identified with IC50s in the low micromolar range. These new inhibitors were found to act via apparently different mechanisms with some inhibiting the reaction in a positively cooperative manner, the inhibition by only some of the compounds was reversed by ADP, and one compound was found to stabilize the enzyme against thermal denaturation. Therefore, these new compounds not only are new leads in the treatment of hyperactive GDH but also are useful in dissecting the complex allosteric nature of the enzyme.

Heterogeneity of CK2 phosphorylation sites in the NS5A protein of different hepatitis C virus genotypes

BACKGROUND/AIMS

The hepatitis C virus NS5A protein is phosphorylated by several cellular kinases, including casein kinase 2 (CK2). Little is known about CK2 phosphorylation of NS5A from different HCV genotypes and clinical isolates.

METHODS

NS5A from patients with HCV-1a (24 cases), HCV-1b (9) or HCV-3 (16) was analyzed by direct sequencing and CK2 phosphorylation sites were defined using a well-validated prediction rule. In vitro phosphorylation assays were performed using recombinant CK2 and synthetic peptides or full-length NS5A. In vivo phosphorylation by endogenous CK2 of NS5A expressed in hepatoma cells was also investigated.

RESULTS

The mean number of CK2 sites within full-length NS5A, was significantly higher in HCV-3 compared to HCV-1a (P<0.01) and HCV-1b (P<0.01). The number of CK2 sites was more homogeneous in HCV-3 variants compared to HCV-1a and HCV-1b variants (P<0.05). The number of predicted CK2 sites correlated with the degree of in vitro and in vivo phosphorylation of NS5A by CK2.

CONCLUSIONS

CK2-dependent phosphorylation of NS5A is heterogeneous among different HCV genotypes and clinical isolates. This might have an influence on virus biology and pathogenicity.

Nucleolin is a receptor that mediates antiangiogenic and antitumor activity of endostatin

The exact molecular mechanism of how endostatin inhibits angiogenesis and tumor growth remains uncharacterized. Here, we report that endostatin specifically binds to the cell surface nucleolin with high affinity. Blockage of nucleolin by a neutralizing antibody or knockdown of nucleolin by the RNA interference results in loss of antiendothelial activities of endostatin. Importantly, a neutralizing antinucleolin antibody abrogates the antiangiogenic and antitumor activities of endostatin in vivo. Nucleolin and endostatin are colocalized on the cell surface of endothelial cells of angiogenic blood vessels in the tumor environment. Finally, we found that endostatin is internalized and transported into cell nuclei of endothelial cell via nucleolin. In the nucleus, the phosphorylation of nucleolin, which is critical for cell proliferation, can be inhibited by endostatin. Our studies demonstrate that nucleolin is a novel functional receptor for endostatin, and mediates the antiangiogenic and antitumor activities of endostatin. These findings also provide mechanistic insights of how endostatin specifically inhibits proliferating endothelial cell growth and angiogenesis.

Differential regulation of mouse equilibrative nucleoside transporter 1 (mENT1) splice variants by protein kinase CK2

Nucleosides are accumulated by cells via a family of equilibrative transport proteins (ENTs). An alternative splice variant of the most common subtype of mouse ENT (ENT1) has been identified which is missing a protein kinase CK2 (casein kinase 2) consensus site (Ser(254)) in the central intracellular loop of the protein. We hypothesized that this variant (mENT1a) would be less susceptible to modulation by CK2-mediated phosphorylation compared to the variant containing the serine at position 254 (mENT1b). Each splice variant was transfected into nucleoside transporter deficient PK15 cells, and stable transfectants assessed for their ability to bind the ENT1-selective probe [(3)H]nitrobenzylthioinosine (NBMPR) and to mediate the cellular uptake of [(3)H]2-chloroadenosine, with or without treatment with the CK2 selective inhibitor, 4,5,6,7-tetrabromobenzotriazole (TBB). mENT1a had a higher affinity for NBMPR relative to mENT1b - measured both directly by the binding of [(3)H]NBMPR, and indirectly via inhibition of [(3)H]2-chloroadenosine influx by NBMPR. Furthermore, incubation of mENT1b-expressing cells with 10 microM TBB for 48 h decreased both the K(D) and B(max) of [(3)H]NBMPR binding, as well as the V(max) of 2-chloroadenosine uptake, whereas similar treatment of mENT1a-expressing cells with TBB had no effect. PK15 cells transfected with hENT1, which has Ser(254), was similar to mENT1b in its response to TBB. In conclusion, inhibition of CK2 activity, or deletion of Ser(254) from mENT1, enhances transporter affinity for the inhibitor, NBMPR, and reduces the number of ENT1 proteins functioning at the level of the plasma membrane.

Protein kinase CK2 phosphorylation of EB2 regulates its function in the production of Epstein-Barr virus infectious viral particles

The Epstein-Barr Virus (EBV) early protein EB2 (also called BMLF1, Mta, or SM) promotes the nuclear export of a subset of early and late viral mRNAs and is essential for the production of infectious virions. We show here that in vitro, protein kinase CK2alpha and -beta subunits bind both individually and, more efficiently, as a complex to the EB2 N terminus and that the CK2beta regulatory subunit also interacts with the EB2 C terminus. Immunoprecipitated EB2 has CK2 activity that phosphorylates several sites within the 80 N-terminal amino acids of EB2, including Ser-55, -56, and -57, which are localized next to the nuclear export signal. EB2S3E, the phosphorylation-mimicking mutant of EB2 at these three serines, but not the phosphorylation ablation mutant EB2S3A, efficiently rescued the production of infectious EBV particles by HEK293(BMLF1-KO) cells harboring an EB2-defective EBV genome. The defect of EB2S3A in transcomplementing 293(BMLF1-KO) cells was not due to impaired nucleocytoplasmic shuttling of the mutated protein but was associated with a decrease in the cytoplasmic accumulation of several late viral mRNAs. Thus, EB2-mediated production of infectious EBV virions is regulated by CK2 phosphorylation at one or more of the serine residues Ser-55, -56, and -57.

A multiplexed protein kinase assay

We report a novel protein kinase assay designed for high-throughput detection of one or many kinases in a complex mixture. A solution-phase phosphorylation reaction is performed on 900 different peptide substrates, each covalently linked to an oligonucleotide tag. After incubation, phosphoserine, phosphothreonine, and phosphotyrosine are chemically labeled, and the substrates are hybridized to a microarray with oligonucleotides complementary to the tags to read out the phosphorylation state of each peptide. Because protein kinases act on more than one peptide sequence, each kinase can be characterized by a unique signature of phosphorylation activity on multiple substrates. Using this method, we determined signatures for 26 purified kinases and demonstrated that enzyme mixtures can be screened for activity and selectivity of inhibition.

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.

The sodium channel Nav1.5a is the predominant isoform expressed in adult mouse dorsal root ganglia and exhibits distinct inactivation properties from the full-length Nav1.5 channel

Nav1.5 is the principal voltage-gated sodium channel expressed in heart, and is also expressed at lower abundance in embryonic dorsal root ganglia (DRG) with little or no expression reported postnatally. We report here the expression of Nav1.5 mRNA isoforms in adult mouse and rat DRG. The major isoform of mouse DRG is Nav1.5a, which encodes a protein with an IDII/III cytoplasmic loop reduced by 53 amino acids. Western blot analysis of adult mouse DRG membrane proteins confirmed the expression of Nav1.5 protein. The Na+ current produced by the Nav1.5a isoform has a voltage-dependent inactivation significantly shifted to more negative potentials (by approximately 5 mV) compared to the full-length Nav1.5 when expressed in the DRG neuroblastoma cell line ND7/23. These results imply that the alternatively spliced exon 18 of Nav1.5 plays a role in channel inactivation and that Nav1.5a is likely to make a significant contribution to adult DRG neuronal function.

Synthesis and biological evaluation of 3-(substituted-benzylidene)-1,3-dihydro-indolin derivatives as human protein kinase CK2 and p60(c-Src) tyrosine kinase inhibitors

Human protein kinase CK2 is an ubiquitous serine/threonine kinase that is typically found in tetrameric complexes consisting of two catalytic (alpha and/or alpha') and two regulatory beta subunits. Although there is growing evidence that besides the participation of CK2 in a complex series of cellular functions, this protein kinase is involved in cell viability, cell proliferation, and neoplastic transformation. In the present study, a series of 3-(substituted-benzylidene)-1,3-dihydro-indolin-2-thione derivatives and the corresponding indolin-2-one congeners were tested for their inhibition of human recombinant protein kinase CK2 in vitro. The efficacy of these compounds was compared with their inhibitory results of p60(c-Src) tyrosine kinase. It was found that 3-(substituted-benzylidene)-1,3-dihydro-indolin-2-thione derivatives are more effective than indolin-2-one congeners for the inhibition of CK2 and p60(c-Src) tyrosine kinase.

Pharmacological inhibition of protein kinase CK2 reverts the multidrug resistance phenotype of a CEM cell line characterized by high CK2 level

Protein kinase CK2 is an ubiquitous and constitutively active kinase, which phosphorylates many cellular proteins and is implicated in the regulation of cell survival, proliferation and transformation. We investigated its possible involvement in the multidrug resistance phenotype (MDR) by analysing its level in two variants of CEM cells, namely S-CEM and R-CEM, normally sensitive or resistant to chemical apoptosis, respectively. We found that, while the CK2 regulatory subunit beta was equally expressed in the two cell variants, CK2alpha catalytic subunit was higher in R-CEM and this was accompanied by a higher phosphorylation of endogenous protein substrates. Pharmacological downregulation of CK2 activity by a panel of specific inhibitors, or knockdown of CK2alpha expression by RNA interference, were able to induce cell death in R-CEM. CK2 inhibitors could promote an increased uptake of chemotherapeutic drugs inside the cells and sensitize them to drug-induced apoptosis in a co-operative manner. CK2 blockade was also effective in inducing cell death of a different MDR line (U2OS). We therefore conclude that inhibition of CK2 can be considered as a promising tool to revert the MDR phenotype.

SIP, a novel ankyrin repeat containing protein, sequesters steroid receptor coactivators in the cytoplasm

Steroid receptor coactivators (SRCs) exert profound effects on animal development and physiology. These coactivators are nuclear proteins and transcription co-regulators that function to facilitate the transcription initiation mediated by nuclear receptors, as well as by other well-known transcription factors. However, how these co-regulators are functionally regulated is poorly understood. During genome-wide screening for SRC-interacting proteins, we identified a novel ankyrin repeat containing protein, SIP (SRC-Interacting Protein), which interacts with SRC coactivators in the cytoplasm. We demonstrated that extracellular stimuli such as the addition of estrogen, induced phosphorylation of SIP in its PEST (Proline, Glutamate, Serine, and Threonine rich) domain by casein kinase II. The phosphorylation of SIP resulted in dissociation of SRC proteins from SIP in the cytoplasm and led to subsequent nuclear translocation of SRC proteins and gene coactivation. Both gain-of-function and loss-of-function experiments indicate that SIP functions to sequester SRC coactivators in the cytoplasm and buffer the availability of these coactivators, thus providing a mechanism for the regulation of the transcription regulators.

Protein Kinase CK2, Cystic Fibrosis Transmembrane Conductance Regulator, and the ΔF508 Mutation

Deletion of phenylalanine 508 (DeltaF508) from the first nucleotide-binding domain (NBD1) of the cystic fibrosis transmembrane conductance regulator (CFTR) is the most common mutation in cystic fibrosis. The F508 region lies within a surface-exposed loop that has not been assigned any interaction with associated proteins. Here we demonstrate that the pleiotropic protein kinase CK2 that controls protein trafficking, cell proliferation, and development binds wild-type CFTR near F508 and phosphorylates NBD1 at Ser-511 in vivo and that mutation of Ser-511 disrupts CFTR channel gating. Importantly, the interaction of CK2 with NBD1 is selectively abrogated by the DeltaF508 mutation without disrupting four established CFTR-associated kinases and two phosphatases. Loss of CK2 association is functionally corroborated by the insensitivity of DeltaF508-CFTR to CK2 inhibition, the absence of CK2 activity in DeltaF508 CFTR-expressing cell membranes, and inhibition of CFTR channel activity by a peptide that mimics the F508 region of CFTR (but not the equivalent DeltaF508 peptide). Disruption of this CK2-CFTR association is the first described DeltaF508-dependent protein-protein interaction that provides a new molecular paradigm in the most frequent form of cystic fibrosis.

Organization and regulation of small conductance Ca2+-activated K+ channel multiprotein complexes

Small conductance Ca2+-activated K+ channels (SK channels) are complexes of four alpha pore-forming subunits each bound by calmodulin (CaM) that mediate Ca2+ gating. Proteomic analysis indicated that SK2 channels also bind protein kinase CK2 (CK2) and protein phosphatase 2A (PP2A). Coexpression of SK2 with the CaM phosphorylation surrogate CaM(T80D) suggested that the apparent Ca2+ sensitivity of SK2 channels is reduced by CK2 phosphorylation of SK2-bound CaM. By using 4,5,6,7-tetrabromo-2-azabenzimidazole, a CK2-specific inhibitor, we confirmed that SK2 channels coassemble with CK2. PP2A also binds to SK2 channels and counterbalances the effects of CK2, as shown by coexpression of a dominant-negative mutant PP2A as well as a mutant SK2 channel no longer able to bind PP2A. In vitro binding studies have revealed interactions between the N and C termini of the channel subunits as well as interactions among CK2 alpha and beta subunits, PP2A, and distinct domains of the channel. In the channel complex, lysine residue 121 within the N-terminal domain of the channel activates SK2-bound CK2, and phosphorylation of CaM is state dependent, occurring only when the channels are closed. The effects of CK2 and PP2A indicate that native SK2 channels are multiprotein complexes that contain constitutively associated CaM, both subunits of CK2, and at least two different subunits of PP2A. The results also show that the Ca2+ sensitivity of SK2 channels is regulated in a dynamic manner, directly through CK2 and PP2A, and indirectly by Ca2+ itself via the state dependence of CaM phosphorylation by CK2.

The human cytomegalovirus virion possesses an activated casein kinase II that allows for the rapid phosphorylation of the inhibitor of NF-kappaB, IkappaBalpha

We documented that the NF-kappaB signaling pathway was rapidly induced following human cytomegalovirus (HCMV) infection of human fibroblasts and that this induced NF-kappaB activity promoted efficient transactivation of the major immediate-early promoter (MIEP). Previously, we showed that the major HCMV envelope glycoproteins, gB and gH, initiated this NF-kappaB signaling event. However, we also hypothesized that there were additional mechanisms utilized by the virus to rapidly upregulate NF-kappaB. In this light, we specifically hypothesized that the HCMV virion contained IkappaBalpha kinase activity, allowing for direct phosphorylation of IkappaBalpha following virion entry into infected cells. In vitro kinase assays performed on purified HCMV virion extract identified bona fide IkappaBalpha kinase activity in the virion. The enzyme responsible for this kinase activity was identified as casein kinase II (CKII), a cellular serine-threonine protein kinase. CKII activity was necessary for efficient transactivation of the MIEP and IE gene expression. CKII is generally considered to be a constitutively active kinase. We suggest that this molecular characteristic of CKII represents the biologic rationale for the viral capture and utilization of this kinase early after infection. The packaging of CKII into the HCMV virion identifies that diverse molecular mechanisms are utilized by HCMV for rapid NF-kappaB activation. We propose that HCMV possesses multiple pathways to increase NF-kappaB activity to ensure that the correct temporal regulation of NF-kappaB occurs following infection and that sufficient threshold levels of NF-kappaB are reached in the diverse array of cells, including monocytes and endothelial cells, infected in vivo.

Protein kinase CK2: a newcomer in the 'druggable kinome'

The acronym CK2 (derived from the misnomer 'casein kinase' 2) denotes one of the most pleiotropic members of the eukaryotic protein kinase superfamily, characterized by an acidic consensus sequence in which a carboxylic acid (or pre-phosphorylated) side chain at position n+3 relative to the target serine/threonine residue plays a crucial role. The latest repertoire of CK2 substrates includes approx. 300 proteins, but the analysis of available phosphopeptide databases from different sources suggests that CK2 alone may be responsible for the generation of a much larger proportion (10-20%) of the eukaryotic phosphoproteome. Although for the time being CK2 is not included among protein kinases whose inhibitors are in clinical practice or in advanced clinical trials, evidence is accumulating that elevated CK2 constitutive activity co-operates to induce a number of pathological conditions, including cancer, infectious diseases, neurodegeneration and cardiovascular pathologies. The development and usage of cell-permeant, selective inhibitors discloses a scenario whereby CK2 plays a global anti-apoptotic role, which under special circumstances may lead to untimely and pathogenic cell survival.

Inhibition of protein kinase CK2 leads to a modulation of androgen receptor dependent transcription in prostate cancer cells

BACKGROUND

The androgen receptor (AR) mediates the biological responses of androgens in the prostate gland. In prostate cancer, this pathway is often deregulated and causes an uncontrolled proliferation.

METHODS

The current study focuses on the effects of an inhibition of protein kinase CK2 on the AR-mediated transcription in LNCaP prostate cancer cells. We used chemical inhibitors of CK2 as well as dominant-negative kinase mutants to downregulate the CK2 activity. We determined the effects of the inhibition by Western blot analysis of endogenous target genes of the AR as well as by reporter assays.

RESULTS

We found that inhibition of CK2 led to a downregulation of the AR-dependent transcription. Moreover, the amount of the AR protein decreased significantly.

CONCLUSION

According to the fact that AR pathways are involved in the development and progression of prostate cancer, the ability to modulate AR function should provide an alternative basis for the development of new cancer therapies.

Coregulated ataxia telangiectasia-mutated and casein kinase sites modulate cAMP-response element-binding protein-coactivator interactions in response to DNA damage

The cyclic AMP-response element-binding protein (CREB) is a bZIP family transcription factor implicated as an oncoprotein and neuron survival factor. CREB is activated in response to cellular stimuli, including cAMP and Ca(2+), via phosphorylation of Ser-133, which promotes interaction between the kinase-inducible domain (KID) of CREB and the KID-interacting domain of CREB-binding protein (CBP). We previously demonstrated that the interaction between CREB and CBP is inhibited by DNA-damaging stimuli through a mechanism whereby CREB is phosphorylated by the ataxia telangiectasia-mutated (ATM) protein kinase. We now show that the ATM phosphorylation sites in CREB are functionally intertwined with a cluster of coregulated casein kinase (CK) sites. We demonstrate that DNA damage-induced phosphorylation of CREB occurs in three steps. The initial event in the CREB phosphorylation cascade is the phosphorylation of Ser-111, which is carried out by CK1 and CK2 under basal conditions and by ATM in response to ionizing radiation. The phosphorylation of Ser-111 triggers the CK2-dependent phosphorylation of Ser-108 and the CK1-dependent phosphorylation of Ser-114 and Ser-117. The phosphorylation of Ser-114 and Ser-117 by CK1 then renders CREB permissive for ATM-dependent phosphorylation on Ser-121. Mutation of Ser-121 alone abrogates ionizing radiation-dependent repression of CREB-CBP complexes, which can be recapitulated using a CK1 inhibitor. Our findings outline a complex mechanism of CREB phosphorylation in which coregulated ATM and CK sites control CREB transactivation potential by modulating its CBP-binding affinity. The coregulated ATM and CK sites identified in CREB may constitute a signaling motif that is common to other DNA damage-regulated substrates.

Evaluation of 3-carboxy-4(1H)-quinolones as inhibitors of human protein kinase CK2

Due to the emerging role of protein kinase CK2 as a molecule that participates not only in the development of some cancers but also in viral infections and inflammatory failures, small organic inhibitors of CK2, besides application in scientific research, may have therapeutic significance. In this paper, we present a new class of CK2 inhibitors-3-carboxy-4(1H)-quinolones. This class of inhibitors has been selected via receptor-based virtual screening of the Otava compound library. It was revealed that the most active compounds, 5,6,8-trichloro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (7) (IC(50) = 0.3 microM) and 4-oxo-1,4-dihydrobenzo[h]quinoline-3-carboxylic acid (9) (IC(50) = 1 microM), are ATP competitive (K(i) values are 0.06 and 0.28 microM, respectively). Evaluation of the inhibitors on seven protein kinases shows considerable selectivity toward CK2. According to theoretical calculations and experimental data, a structural model describing the key features of 3-carboxy-4(1H)-quinolones responsible for tight binding to CK2 active site has been developed.

The DEK nuclear autoantigen is a secreted chemotactic factor

The nuclear DNA-binding protein DEK is an autoantigen that has been implicated in the regulation of transcription, chromatin architecture, and mRNA processing. We demonstrate here that DEK is actively secreted by macrophages and is also found in synovial fluid samples from patients with juvenile arthritis. Secretion of DEK is modulated by casein kinase 2, stimulated by interleukin-8, and inhibited by dexamethasone and cyclosporine A, consistent with a role as a proinflammatory molecule. DEK is secreted in both a free form and in exosomes, vesicular structures in which transcription-modulating factors such as DEK have not previously been found. Furthermore, DEK functions as a chemotactic factor, attracting neutrophils, CD8+ T lymphocytes, and natural killer cells. Therefore, the DEK autoantigen, previously described as a strictly nuclear protein, is secreted and can act as an extracellular chemoattractant, suggesting a direct role for DEK in inflammation.

Casein kinase 2 (CK2) increases survivin expression via enhanced beta-catenin-T cell factor/lymphoid enhancer binding factor-dependent transcription

Increased expression of casein kinase 2 (CK2) is associated with hyperproliferation and suppression of apoptosis in cancer. Mutations in the tumor suppressor APC (adenomatous polyposis coli) are frequent in colon cancer and often augment beta-catenin-T cell factor (Tcf)/lymphoid enhancer binding factor (Lef)-dependent transcription of genes such as c-myc and cyclin-D1. CK2 has also been implicated recently in the regulation of beta-catenin stability. To identify mechanisms by which CK2 promotes survival, effects of the specific CK2 inhibitors 4,5,6,7-tetrabromobenzotriazole (TBB) and 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole were assessed. TBB and 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole significantly decreased proliferation and increased apoptosis of HT29(US) colon cancer cells. RT-PCR and immunoblot analysis revealed that both inhibitors decreased survivin mRNA and protein levels in HT29(US) cells. Similar effects were observed with TBB in human DLD-1 and SW-480 colorectal cells as well as ZR-75 breast cancer cells and HEK-293T embryonic kidney cells. Expression of GFP-CK2alpha in HEK-293T cells resulted in beta-catenin-Tcf/Lef-dependent up-regulation of survivin and increased resistance to anticancer drugs. Augmented beta-catenin-Tcf/Lef-dependent transcription and resistance to apoptosis observed upon GFP-CK2alpha expression were abolished by TBB. Alternatively, HEK-293T cells expressing GFP-survivin were resistant to TBB-induced apoptosis. Finally, siRNA-mediated down-regulation of CK2alpha in HEK-293T cells coincided with reduced beta-catenin and survivin levels. Taken together, these results suggest that CK2 kinase activity promotes survival by increasing survivin expression via beta-catenin-Tcf/Lef-mediated transcription. Hence, selective CK2 inhibition or down-regulation in tumors may provide an attractive opportunity for the development of novel cancer therapies.

GTP binding and hydrolysis kinetics of human septin 2

Septins are a family of conserved proteins that are essential for cytokinesis in a wide range of organisms including fungi, Drosophila and mammals. In budding yeast, where they were first discovered, they are thought to form a filamentous ring at the bridge between the mother and bud cells. What regulates the assembly and function of septins, however, has remained obscure. All septins share a highly conserved domain related to those found in small GTPases, and septins have been shown to bind and hydrolyze GTP, although the properties of this domain and the relationship between polymerization and GTP binding/hydrolysis is unclear. Here we show that human septin 2 is phosphorylated in vivo at Ser218 by casein kinase II. In addition, we show that recombinant septin 2 binds guanine nucleotides with a Kd of 0.28 microm for GTPgammaS and 1.75 microm for GDP. It has a slow exchange rate of 7 x 10(-5) s(-1) for GTPgammaS and 5 x 10(-4) s(-1) for GDP, and an apparent kcat value of 2.7 x 10(-4) s(-1), similar to those of the Ras superfamily of GTPases. Interestingly, the nucleotide binding affinity appears to be altered by phosphorylation at Ser218. Finally, we show that a single septin protein can form homotypic filaments in vitro, whether bound to GDP or GTP.

Casein kinase 2 associates with initiation-competent RNA polymerase I and has multiple roles in ribosomal DNA transcription

Mammalian RNA polymerase I (Pol I) complexes contain a number of associated factors, some with undefined regulatory roles in transcription. We demonstrate that casein kinase 2 (CK2) in human cells is associated specifically only with the initiation-competent Pol Ibeta isoform and not with Pol Ialpha. Chromatin immunoprecipitation analysis places CK2 at the ribosomal DNA (rDNA) promoter in vivo. Pol Ibeta-associated CK2 can phosphorylate topoisomerase IIalpha in Pol Ibeta, activator upstream binding factor (UBF), and selectivity factor 1 (SL1) subunit TAFI110. A potent and selective CK2 inhibitor, 3,8-dibromo-7-hydroxy-4-methylchromen-2-one, limits in vitro transcription to a single round, suggesting a role for CK2 in reinitiation. Phosphorylation of UBF by CK2 increases SL1-dependent stabilization of UBF at the rDNA promoter, providing a molecular mechanism for the stimulatory effect of CK2 on UBF activation of transcription. These positive effects of CK2 in Pol I transcription contrast to that wrought by CK2 phosphorylation of TAFI110, which prevents SL1 binding to rDNA, thereby abrogating the ability of SL1 to nucleate preinitiation complex (PIC) formation. Thus, CK2 has the potential to regulate Pol I transcription at multiple levels, in PIC formation, activation, and reinitiation of transcription.

CK2-mediated stimulation of Pol I transcription by stabilization of UBF-SL1 interaction

High levels of rRNA synthesis by RNA polymerase I are important for cell growth and proliferation. In vitro studies have indicated that the formation of a stable complex between the HMG box factor [Upstream binding factor (UBF)] and SL1 at the rRNA gene promoter is necessary to direct multiple rounds of Pol I transcription initiation. The recruitment of SL1 to the promoter occurs through protein interactions with UBF and is regulated by phosphorylation of UBF. Here we show that the protein kinase CK2 co-immunoprecipitates with the Pol I complex and is associated with the rRNA gene promoter. Inhibition of CK2 kinase activity reduces Pol I transcription in cultured cells and in vitro. Significantly, CK2 regulates the interaction between UBF and SL1 by counteracting the inhibitory effect of HMG boxes five and six through the phosphorylation of specific serines located at the C-terminus of UBF. Transcription reactions with immobilized templates indicate that phosphorylation of CK2 phosphoacceptor sites in the C-terminal domain of UBF is important for promoting multiple rounds of Pol I transcription. These data demonstrate that CK2 is recruited to the rRNA gene promoter and directly regulates Pol I transcription re-initiation by stabilizing the association between UBF and SL1.

v-Src-dependent Down-regulation of the Ste20-like Kinase SLK by Casein Kinase II

We have previously shown that the Ste20-like kinase SLK is a microtubule-associated protein inducing actin stress fiber disassembly. Here, we show that v-Src expression can down-regulate SLK activity. This down-regulation is independent of focal adhesion kinase but requires v-Src kinase activity and membrane translocation. SLK down-regulation by v-Src is indirect and is accompanied by SLK hyperphosphorylation on serine residues. Deletion analysis revealed that casein kinase II (CK2) sites at position 347/348 are critical for v-Src-dependent modulation of SLK activity. Further studies show that CK2 can directly phosphorylate SLK at these positions and that inhibition of CK2 in v-Src-transformed cells results in normal kinase activity. Finally, CK2 and SLK can be co-localized in fibroblasts spreading on fibronectin-coated substrates, suggesting a mechanism whereby SLK may be regulated at sites of actin remodeling, such as membrane lamellipodia and ruffles, through CK2.

Multiple myeloma cell survival relies on high activity of protein kinase CK2

Casein kinase 2 (CK2) is a ubiquitous cellular serine-threonine kinase that regulates relevant biologic processes, many of which are dysregulated in malignant plasma cells. Here we investigated its role in multiple myeloma (MM). Analysis of MM cell lines and highly purified malignant plasma cells in patients with MM revealed higher protein and CK2 activity levels than in controls (normal in vitro-generated polyclonal plasma cells and B lymphocytes). The inhibition of CK2 with specific synthetic compounds or by means of RNA interference caused a cytotoxic effect on MM plasma cells that could not be overcome by IL-6 or IGF-I and that was associated with the activation of extrinsic and intrinsic caspase cascades. CK2 blockage lowered the sensitivity threshold of MM plasma cells to the cytotoxic effect of melphalan. CK2 inhibition also resulted in impaired IL-6-dependent STAT3 activation and in decreased basal and TNF-alpha-dependent I kappaB alpha degradation and NF-kappaB-driven transcription. Our data show that CK2 was involved in the pathophysiology of MM, suggesting that it might play a crucial role in controlling survival and sensitivity to chemotherapeutics of malignant plasma cells.

Serine-phosphorylated STAT1 is a prosurvival factor in Wilms' tumor pathogenesis

Wilms' tumor (WT), one of the most common pediatric solid cancers, arises in the developing kidney as a result of genetic and epigenetic changes that lead to the abnormal proliferation and differentiation of the metanephric blastema. As activation of signal transducers and activators of transcription (STATs) plays an important role in the maintenance/growth and differentiation of the metanephric blastema, and constitutively activated STATs facilitate neoplastic behaviors of a variety of cancers, we hypothesized that dysregulation of STAT signaling may also contribute to WT pathogenesis. Accordingly, we evaluated STAT phosphorylation patterns in tumors and found that STAT1 was constitutively phosphorylated on serine 727 (S727) in 19 of 21 primary WT samples and two WT cell lines. An inactivating mutation of S727 to alanine reduced colony formation of WT cells in soft agar by more than 80% and induced apoptosis under conditions of growth stress. S727-phosphorylated STAT1 provided apoptotic resistance for WT cells via upregulation of expression of the heat-shock protein (HSP)27 and antiapoptotic protein myeloid cell leukemia (MCL)-1. The kinase responsible for STAT1 S727 phosphorylation in WT cells was identified based upon the use of selective inhibitors as protein kinase CK2, not p38, MAP-kinase kinase (MEK)1/2, phosphatidylinositol 3'-kinase, protein kinase C or Ca/calmodulin-dependent protein kinase II (CaMKII). The inhibition of CK2 blocked the anchorage-independent growth of WT cells and induced apoptosis under conditions of growth stress. Our findings suggest that serine-phosphorylated STAT1, as a downstream target of protein kinase CK2, plays a critical role in the pathogenesis of WT and possibly other neoplasms with similar STAT1 phosphorylation patterns.

Development and exploitation of CK2 inhibitors

A number of quite specific and fairly potent inhibitors of protein kinase CK2, belonging to the classes of condensed polyphenolic compounds, tetrabromobenzimidazole/triazole derivatives and indoloquinazolines are available to date. The structural basis for their selectivity is provided by a hydrophobic pocket adjacent to the ATP/GTP binding site, which in CK2 is smaller than in the majority of other protein kinases due to the presence of a number of residues whose bulky side chains are generally replaced by smaller ones. Consequently a doubly substituted CK2 mutant V66A,I174A is much less sensitive than CK2 wild type to these classes of inhibitors. The most efficient inhibitors both in terms of potency and selectivity are 4,5,6,7-tetrabromo-1H-benzotriazole, TBB (Ki = 0.4 microM), the TBB derivative 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole, DMAT (Ki = 0.040 microM), the emodin related coumarinic compound 8-hydroxy-4-methyl-9-nitrobenzo[g]chromen-2-one, NBC (Ki = 0.22 microM) and the indoloquinazoline derivative ([5-oxo-5,6-dihydroindolo-(1,2a)quinazolin-7-yl]acetic acid), IQA (Ki = 0.17 microM). These inhibitors are cell permeable as judged from ability to block CK2 in living cells and they have been successfully employed, either alone or in combination with CK2 mutants refractory to inhibition, to dissect signaling pathways affected by CK2 and to identify the endogenous substrates of this pleitropic kinase. By blocking CK2 these inhibitors display a remarkable pro-apoptotic efficacy on a number of tumor derived cell lines, a property which can be exploited in perspective to develop antineoplastic drugs.

Protein kinase CK2 phosphorylates the Nef protein from a neurovirulent simian immunodeficiency virus

The Nef protein of Human Immunodeficiency Virus (HIV) and Simian Immunodeficiency Virus (SIV) is a pluripotent accessory protein that plays a critical role in disease progression. One analogous characteristic of Nef proteins from SIV and HIV is the ability to associate with cellular kinases. We have previously reported that the Nef protein from a macrophage-tropic neurovirulent SIV clone, SIV/17E-Fr, is associated with an unknown kinase activity that is distinct from the p21-associated kinase that interacts with SIVmac239 Nef. Using site-directed mutagenesis and kinase-specific inhibitors, we have identified this kinase as the ubiquitous serine/threonine kinase, protein kinase CK2.

A protein kinase CK2 site flanking the nuclear targeting signal enhances nuclear transport of human cytomegalovirus ppUL44

The processivity factor of the human cytomegalovirus (HCMV) DNA polymerase phosphoprotein ppUL44 plays an essential role in viral replication, showing nuclear localization in infected cells. The present study examines ppUL44's nuclear import pathway for the first time, ectopic expression of ppUL44 revealing a strong nuclear localization in transfected COS-7 and other cell types, implying that no other HCMV proteins are required for nuclear transportation and retention. We show that of the two potential nuclear localization signals (NLSs) located at amino acids 162-168 (NLS1) and 425-431 (NLS2), NLS2 is necessary and sufficient to confer nuclear localization. Moreover, using enzyme-linked immunosorbent assays and gel mobility shift assays, we show that NLS2 is recognized with high affinity by the importin (IMP) alpha/beta heterodimer. Using gel mobility shift and transient transfection assays, we find that flanking sequences containing a cluster of potential phosphorylation sites, including a consensus site for protein kinase CK2 (CK2) at Ser413 upstream of the NLS, increase NLS2-dependent IMP binding and nuclear localization, suggesting a role for these sites in enhancing UL44 nuclear transport. Results from site-directed mutagenic analysis and live-cell imaging of green fluorescent protein (GFP)-UL44 fusion protein-expressing cells treated with the CK2-specific inhibitor 4,5,6,7-tetrabromobenzotriazole are consistent with phosphorylation of Ser413 enhancing ppUL44 nuclear transport.

Inspecting the structure-activity relationship of protein kinase CK2 inhibitors derived from tetrabromo-benzimidazole

CK2 is a very pleiotropic protein kinase whose high constitutive activity is suspected to cooperate to neoplasia. Here, the crystal structure of the complexes between CK2 and three selective tetrabromo-benzimidazole derivatives inhibiting CK2 with Ki values between 40 and 400 nM are presented. The ligands bind to the CK2 active site in a different way with respect to the parent compound TBB. They enter more deeply into the cavity, establishing halogen bonds with the backbone of Glu114 and Val116 in the hinge region. A detailed analysis of the interactions highlights a major role of the hydrophobic effect in establishing the rank of potency within this class of inhibitors and shows that polar interactions are responsible for the different orientation of the molecules in the active site.

Phosphorylation on Ser106 Modulates the Cellular Functions of the SHOX Homeodomain Protein

Mutations within the homeobox SHOX gene have been associated with short stature and the skeletal deformities found in Léri-Weill, Turner and Langer syndromes implying an involvement of SHOX in growth and bone formation. Despite its clinical significance, the precise role of SHOX and the mechanisms that modulate its functions remain unknown. We reported previously that SHOX is a nuclear protein that specifically binds DNA and acts as a transcriptional activator. We have shown that ectopic expression of SHOX leads to cell-cycle arrest and apoptosis in osteosarcoma and primary cells. To further characterize SHOX, we investigated whether the protein could be a target for phosphorylation. Here, we report that SHOX is phosphorylated exclusively on serine residues in vivo. Two-dimensional phospho-peptide mapping showed that SHOX is phosphorylated to various extents on multiple sites. Site-directed mutagenesis demonstrated that serine 106 is the major SHOX phosphorylation site. We show also that casein kinase II phosphorylates SHOX on serine 106 efficiently in vitro and specific casein kinase II inhibitors reduce SHOX phosphorylation strongly in vivo. Finally, we provide evidence that phosphorylation may play an important role in modulating SHOX biological activities, since a S106A SHOX mutant, defective in phosphorylation, does not activate transcription and fails to induce cell-cycle arrest and apoptosis.

Independent role of phosphoinositol-3-kinase (PI3K) and casein kinase II (CK-2) in EGFR and Her-2-mediated constitutive NF-kappaB activation in prostate cancer cells

BACKGROUND

Recent research has highlighted the potential role of EGFR and Her-2 in the constitutive activation of NF-kappaB (NF-kappaB) in prostate cancer cells, although the mechanism by which these receptors activate NF-kappaB in these cells remains unclear.

METHODS AND RESULTS

Using pharmacological and genetic approaches we show that in PC-3 cells, EGFR and Her-2 are involved in the constitutive activation of NF-kappaB through two different mechanisms. EGFR activates NF-kappaB through the PI3K/Akt pathway that leads to the phosphorylation of IkappaBalpha on serines 32 and 36, thereby promoting the nuclear translocation of the p65 subunit. In contrast, Her-2 activates NF-kappaB through Casein Kinase II (CK-2) activation independently of IkappaBalpha phosphorylation on serines 32 and 36.

CONCLUSIONS

Our study not only directly clarifies the signaling pathways involved in NF-kappaB activation in prostate cancer cell lines and but also provides a framework for further studies in the clinical characterization and management of prostate cancer.

CK2 regulates in vitro the activity of the yeast cyclin-dependent kinase inhibitor Sic1

We have previously demonstrated that the cyclin-dependent kinase inhibitor (Cki) Sic1 of Saccharomyces cerevisiae is phosphorylated in vitro by the CK2 kinase on Ser(201) residue. Moreover, we have collected evidence showing that Sic1 is functionally and structurally related to mammalian Cki p27(Kip1) and binds to the mammalian Cdk2/cyclin A complex with a similar mode of inhibition. In this paper, we use SPR analysis to investigate the binding of Sic1 to the catatytic and regulatory subunits of CK2. Evidence is presented showing that phosphorylation of Sic1 at the CK2 consensus site QES(201)EDEED increases the binding of a Sic1-derived peptide to the Cdk2/cyclin A complex, a functional homologue of the yeast Cdk1/Clb5,6. Moreover, Sic1 fully phosphorylated in vitro on Ser(201) by CK2 is shown to be a stronger inhibitor of the Cdk/cyclin complexes than the unphosphorylated protein. Taken together, these data disclose the possibility that CK2 plays a role in the regulation of Sic1 activity.

Protein kinase CK2 phosphorylates and upregulates Akt/PKB

Treatment of Jurkat cells with specific inhibitors of protein kinase CK2 induces apoptosis. Here we provide evidence that the anti-apoptotic effect of CK2 can be at least partially mediated by upregulation of the Akt/PKB pathway. Such a conclusion is based on the following observations: (1) inhibition of CK2 by cell treatment with two structurally unrelated CK2 inhibitors induces downregulation of Akt/PKB, as judged from decreased phosphorylation of its physiological targets, and immunoprecipitate kinase assay; (2) similar results are observed upon reduction of CK2 catalytic subunit by the RNA-interference technique; (3) Akt/PKB Ser129 is phosphorylated by CK2 in vitro and in vivo; (4) such a phosphorylation of activated Akt/PKB correlates with a further increase in catalytic activity. These data disclose an unanticipated mechanism by which constitutive phosphorylation by CK2 may be required for maximal activation of Akt/PKB.