Inhibition of cyclin-dependent kinases by purine analogues

Anti-angiogenic potential of small molecular inhibitors of cyclin dependent kinases in vitro

Small molecular kinase inhibitors are promising novel drugs. Initially, they were designed for the highest possible specificity. Recently, this concept has been challenged by multikinase inhibitors, which are clinically more potent. This change of paradigm calls for re-examination of already known compounds in different functional contexts. We have compared 6 reported structurally different inhibitors of cyclin-dependent kinases (Cdks) regarding their functional effects on endothelial cells (proliferation, cell cycle, apoptosis, migration, tube formation), as well as their actions on some kinases (AKT, p38, ERK1/2, c-src, GSK3β). Only some of these compounds had anti-angiogenic effects in concentrations up to 10 μM (aminopurvalanol, indirubin-3'-monoxime, and alsterpaullone), depending on their kinase profile. Interestingly, the impact of the compounds on Cdks seemed to be of minor importance, as compared to other mechanisms. Aminopurvalanol, indirubin-3'-monoxime, and alsterpaullone might turn out as interesting scaffolds for the development of novel anti-angiogenic drugs.

S phase entry causes homocysteine-induced death while ataxia telangiectasia and Rad3 related protein functions anti-apoptotically to protect neurons

A major phenotype seen in neurodegenerative disorders is the selective loss of neurons due to apoptotic death and evidence suggests that inappropriate re-activation of cell cycle proteins in post-mitotic neurons may be responsible. To investigate whether reactivation of the G1 cell cycle proteins and S phase entry was linked with apoptosis, we examined homocysteine-induced neuronal cell death in a rat cortical neuron tissue culture system. Hyperhomocysteinaemia is a physiological risk factor for a variety of neurodegenerative diseases, including Alzheimer's disease. We found that in response to homocysteine treatment, cyclin D1, and cyclin-dependent kinases 4 and 2 translocated to the nucleus, and p27 levels decreased. Both cyclin-dependent kinases 4 and 2 regained catalytic activity, the G1 gatekeeper retinoblastoma protein was phosphorylated and DNA synthesis was detected, suggesting transit into S phase. Double-labelling immunofluorescence showed a 95% co-localization of anti-bromodeoxyuridine labelling with apoptotic markers, demonstrating that those cells that entered S phase eventually died. Neurons could be protected from homocysteine-induced death by methods that inhibited G1 phase progression, including down-regulation of cyclin D1 expression, inhibition of cyclin-dependent kinases 4 or 2 activity by small molecule inhibitors, or use of the c-Abl kinase inhibitor, Gleevec, which blocked cyclin D and cyclin-dependent kinase 4 nuclear translocation. However, blocking cell cycle progression post G1, using DNA replication inhibitors, did not prevent apoptosis, suggesting that death was not preventable post the G1-S phase checkpoint. While homocysteine treatment caused DNA damage and activated the DNA damage response, its mechanism of action was distinct from that of more traditional DNA damaging agents, such as camptothecin, as it was p53-independent. Likewise, inhibition of the DNA damage sensors, ataxia-telangiectasia mutant and ataxia telangiectasia and Rad3 related proteins, did not rescue apoptosis and in fact exacerbated death, suggesting that the DNA damage response might normally function neuroprotectively to block S phase-dependent apoptosis induction. As cell cycle events appear to be maintained in vivo in affected neurons for weeks to years before apoptosis is observed, activation of the DNA damage response might be able to hold cell cycle-induced death in check.

Probing the probes: fitness factors for small molecule tools

Chemical probes for interrogating biological processes are of considerable current interest. Cell permeable small molecule tools have a major role in facilitating the functional annotation of the human genome, understanding both physiological and pathological processes, and validating new molecular targets. To be valuable, chemical tools must satisfy necessary criteria and recent publications have suggested objective guidelines for what makes a useful chemical probe. Although recognizing that such guidelines may be valuable, we caution against overly restrictive rules that may stifle innovation in favor of a "fit-for-purpose" approach. Reviewing the literature and providing examples from the cancer field, we recommend a series of "fitness factors" to be considered when assessing chemical probes. We hope this will encourage innovative chemical biology research while minimizing the generation of poor quality and misleading biological data, thus increasing understanding of the particular biological area, to the benefit of basic research and drug discovery.

Cytochalasin D, LY294002 and olomoucine synergize in promoting death of melanoma cells through activation of caspase-3 and apoptosis

Many of the current anticancer therapies rely on the induction of apoptosis, and several mechanisms that protect cells against apoptosis may be upregulated in tumors. A growing body of evidence suggests that single drugs with a clearly defined intracellular target may be less efficient in arresting tumor growth and induction of apoptosis than multitargeted strategies. To prove that this is also the case for melanoma, we studied five cell lines, which represent different stages of tumor progression. We tested cell viability, terminal dUTP nick-end labeling and activation of caspase-3 upon exposure to cytochalasin D, LY294002 and olomoucine, added either alone or in various combinations. The obtained data were compared with effects caused by staurosporine. The results show that whereas staurosporine efficiently induced apoptosis in all tested melanoma cell lines, the other drugs had only moderate effects when administered alone. In contrast, the combinations of drugs were more effective in inducing caspase-3 activity and reducing cell viability. In particular, the triple combination of cytochalasin D+LY294002+olomoucine was almost as effective as staurosporine in inducing caspase-3 activity and apoptosis. These results prove that it is possible to design new pharmacological strategies that will effectively induce caspase-3 activity and apoptosis in melanoma. The possible explanations of the observed synergy between the tested drugs are also discussed.

Trisubstituted purines are useful tools for developing potent plant mitogen-activated protein kinase inhibitors

A number of compounds have been reported to be specific inhibitors of protein kinases mediated by structural-based selectivity, but the development of specific inhibitors has not yet been addressed in plant science. Here we tested C2, C6, and N9-trisubstituted purines to determine the basic relationship between their chemical structure and inhibitory activity versus a plant mitogen-activated protein kinase (MAPK). Modification of substitution at positions C2 and N9 caused increased inhibitory activity of 6-benzylaminopurine analogs. In the case of 6-isopentenyladenine derivatives, the addition of a methyl group at position N9 caused at least 2-fold increased inhibitory activity, as compared with the addition of an isopropyl group. The data indicate that the selectivity and potency of inhibitors can be improved by modification of the chemical structure, suggesting that trisubstituted purines are powerful tools for probing biological processes and understanding the physiological roles of MAPK signaling.

Seliciclib in malignancies

Cyclins and cyclin-dependent kinases (CDK) form a key part of the regulatory proteins that govern the cell cycle. Aberrancy in their function can lead to uncontrolled growth and proliferation of the cells which forms the basis of many human diseases, especially cancers. Seliciclib (CYC202, R-roscovitine) is a second-generation CDK inhibitor that competes for ATP binding sites on these kinases, reducing tumor growth and inducing cell death. It is a direct inhibitor of cyclin E/CDK2 and also has inhibitory effects on cyclin H/CDK7 and cyclin T/CDK9. Seliciclib leads to growth arrest and apoptosis of cell lines through activation of the p53 gene, inhibition of RNA processing and blockage of the RNA polymerase II-dependent transcription, and reduction of anti-apoptotic proteins. Seliciclib has good oral bioavailability, although its absorption is slowed by food. It is distributed rapidly to the body tissues and metabolized rapidly to a carboxylated derivative that is excreted by the kidneys. The major adverse effects of seliciclib are electrolyte disturbances (hypokalemia, hyponatremia), gastrointestinal side effects (nausea, emesis, anorexia), fatigue, transient hyperglycemia, elevation of liver enzymes and reversible elevation of serum creatinine. At present, it is in Phase II trials for non-small cell lung cancer and nasopharyngeal carcinoma.

The inhibitor of cyclin-dependent kinases, olomoucine II, exhibits potent antiviral properties

BACKGROUND

Olomoucine II, the most recent derivative of roscovitine, is an exceptionally potent pharmacological inhibitor of cyclin-dependent kinase activities. Here, we report that olomoucine II is also an effective antiviral agent.

METHODS

Antiviral activities of olomoucine II were tested on a range of human viruses in in vitro assays that evaluated viral growth and replication.

RESULTS

Olomoucine II inhibited replication of a broad range of wild-type human viruses, including herpes simplex virus, human adenovirus type-4 and human cytomegalovirus. Olomoucine II also inhibited replication of vaccinia virus and herpes simplex virus mutants resistant to conventional acyclovir treatment. This report is the first demonstration of a poxvirus being sensitive to a cyclin-dependent kinase inhibitor. The antiviral effects of olomoucine II could be observed at lower concentrations than with roscovitine, although both were short-term. A remarkable observation was that olomoucine II, when used in combination with the DNA polymerase inhibitor cidofovir, was able to almost completely eliminate the spread of infectious adenovirus type-4 progeny from infected cells.

CONCLUSIONS

Our results show that when targeting two complementary antiviral mechanisms, strongly additive effects could be observed.

6-Substituted 2-(N-trifluoroacetylamino)imidazopyridines induce cell cycle arrest and apoptosis in SK-LU-1 human cancer cell line

A series of 6-substituted 2-(N-trifluoroacetylamino)imidazopyridines have been synthesized and their bioactivities were evaluated. Compounds 6a, 6c, and 11a were the most active compounds with modest cytotoxic activity against six human cancer cell lines U251 (glioma), PC-3 (prostate), K-562 (leukemia), HCT-15 (colon), MCF7 (breast) and SK-LU-1 (lung). The cell cycle analysis showed that compounds 6a, 6c, and 11a induce a G2/M phase cell cycle arrest on SK-LU-1 cell line where inhibition of CDK-1 and CDK-2 may be implicated.

Radiosynthesis and radiopharmacological evaluation of cyclin-dependent kinase 4 (Cdk4) inhibitors

Tumor cells are characterized by their loss of growth control resulting from alterations in regulating pathways of the cell cycle, such as a deregulated cyclin-dependent kinase (Cdk) activity and/or Cdk expression. Appropriately radiolabeled Cdk4 inhibitors are discussed as promising molecular probes for imaging cell proliferation processes and tumor visualization by PET. This work describes the design, synthesis and radiopharmacological evaluation of two (124)I-labeled Cdk4 inhibitors as potential radiotracers for imaging of Cdk4 in vivo. Treatment of a solution containing labeling precursors with [(124)I]NaI gave radiolabeled Cdk4 inhibitors [(124)I]CKIA and [(124)I]CKIB in radiochemical yields of up to 35%. (124)I-labeled radiotracers [(124)I]CKIA and [(124)I]CKIB were used in cell uptake studies as well as biodistribution studies in Wistar rats and small-animal PET in tumor-bearing mice. In vitro radiotracer uptake studies in adherent tumor cells using [(124)I]CKIA showed substantial uptake in HT-29 and FaDu cells (750-850 %ID/mg protein [(124)I]CKIA and 900-1000 %ID/mg protein [(124)I]CKIB) after 1 h at 37 degrees C. Biodistribution of [(124)I]CKIA and [(124)I]CKIB showed rapid blood clearance of radioactivity and an accumulation as well as metabolization in the liver. Both radiotracers were administered intravenously to mouse FaDu xenograft tumor model and imaging studies were performed on a small-animal PET scanner. Both imaging techniques showed only little uptake of both radiotracers in the FaDu tumor xenografts.

Cyclin-dependent kinase 5 inhibitors: inhibition of dopamine transporter activity

Cyclin-dependent kinase (Cdk) 5 reduces the rewarding properties of psychostimulants by dampening postsynaptic dopamine (DA) receptor signaling. Cdk5 is also present in midbrain DA neurons, where the DA transporter (DAT) is localized and limits DA neurotransmission by removing extracellular DA. Here, we tested the hypothesis that Cdk5 could also affect the disposition of DA by regulating DAT activity. Incubation of rat dorsal striatal (dSTR) synaptosomes with the Cdk5 inhibitors roscovitine, olomoucine, and 4-{[(7-oxo-6,7-dihydro-8H-[1,3]thiazolo[5,4-e]indol-8-ylidene)methyl]amino}-N-(2-pyridyl)benzenesulfonamide (GW8510) or the inactive congener iso-olomoucine resulted in a rapid, concentration-dependent inhibition of specific [3H]DA uptake. However, roscovitine was the only inhibitor that did not also decrease [3H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (WIN35,428) binding to dSTR DATs. Roscovitine-induced inhibition of dSTR [3H]DA uptake was explained by decreased maximal uptake velocity, without a change in cell-surface DAT levels. Roscovitine did not enhance [3H]DA release mediated by either DAT reverse-transport or Ca(2+) channels in dSTR slices. Instead, roscovitine enhanced spontaneous [3H]DA outflow and inhibited DAT-mediated [3H]DA reaccumulation into dSTR slices. To explore the involvement of Cdk5 in roscovitine-induced down-regulation of DAT activity, Cdk5 protein was knocked down via Cdk5-small interfering RNA by as much as 86% in porcine aortic endothelial cells stably expressing human (h)DATs. However, Cdk5 depletion did not alter hDAT activity. Taken together, our results suggest that roscovitine inhibits DAT activity independently of Cdk5; therefore, results obtained with such inhibitors should be interpreted with caution. Our study is the first to demonstrate that Cdk5 inhibitors reduce brain DAT activity via a mechanism that is independent of DAT trafficking and reverse-transport.

Rottlerin inhibits (Na+, K+)-ATPase activity in brain tissue and alters D-aspartate dependent redistribution of glutamate transporter GLAST in cultured astrocytes

The naturally occurring toxin rottlerin has been used by other laboratories as a specific inhibitor of protein kinase C-delta (PKC-delta) to obtain evidence that the activity-dependent distribution of glutamate transporter GLAST is regulated by PKC-delta mediated phosphorylation. Using immunofluorescence labelling for GLAST and deconvolution microscopy we have observed that D-aspartate-induced redistribution of GLAST towards the plasma membranes of cultured astrocytes was abolished by rottlerin. In brain tissue in vitro, rottlerin reduced apparent activity of (Na+, K+)-dependent ATPase (Na+, K+-ATPase) and increased oxygen consumption in accordance with its known activity as an uncoupler of oxidative phosphorylation ("metabolic poison"). Rottlerin also inhibited Na+, K+-ATPase in cultured astrocytes. As the glutamate transport critically depends on energy metabolism and on the activity of Na+, K+-ATPase in particular, we suggest that the metabolic toxicity of rottlerin and/or the decreased activity of the Na+, K+-ATPase could explain both the glutamate transport inhibition and altered GLAST distribution caused by rottlerin even without any involvement of PKC-delta-catalysed phosphorylation in the process.

Strong inhibition of replicative DNA synthesis in the developing rat cerebral cortex and glioma cells by roscovitine

The effects of the cyclin-dependent kinase inhibitors roscovitine and olomoucine on DNA synthesis rate during normal rat brain development were studied by using short time (90 min) incubation. Both purine analogues at 100 microM concentration decreased the DNA synthesis of rat cerebral cortex in an age-dependent manner. The maximum inhibitory effect (approximately 90% for roscovitine, approximately 60% for olomoucine) occurred in rats of 2-13 days postnatal age. In adult rats (> 60 days postnatal age), the effect of both purine analogues was low. Roscovitine even at 200 microM concentration did not inhibit the fraction of DNA synthesis insensitive to hydroxyurea (unscheduled DNA synthesis (UDS)). In addition, in the RG2 rat glioma model, roscovitine produced a strong inhibition of DNA synthesis in glioma cells when compared to adult normal tissue. Since in adult rat brain more than 60% of DNA synthesis is related to DNA repair, usually measured as UDS, our results indicate that roscovitine strongly blocks ongoing DNA synthesis connected with replicative processes.

Identifying the proteins to which small-molecule probes and drugs bind in cells

Most small-molecule probes and drugs alter cell circuitry by interacting with 1 or more proteins. A complete understanding of the interacting proteins and their associated protein complexes, whether the compounds are discovered by cell-based phenotypic or target-based screens, is extremely rare. Such a capability is expected to be highly illuminating--providing strong clues to the mechanisms used by small-molecules to achieve their recognized actions and suggesting potential unrecognized actions. We describe a powerful method combining quantitative proteomics (SILAC) with affinity enrichment to provide unbiased, robust and comprehensive identification of the proteins that bind to small-molecule probes and drugs. The method is scalable and general, requiring little optimization across different compound classes, and has already had a transformative effect on our studies of small-molecule probes. Here, we describe in full detail the application of the method to identify targets of kinase inhibitors and immunophilin binders.

Mengovirus-induced rearrangement of the nuclear pore complex: hijacking cellular phosphorylation machinery

Representatives of several picornavirus genera have been shown previously to significantly enhance non-controllable bidirectional exchange of proteins between nuclei and cytoplasm. In enteroviruses and rhinoviruses, enhanced permeabilization of the nuclear pores appears to be primarily due to proteolytic degradation of some nucleoporins (protein components of the pore), whereas this effect in cardiovirus-infected cells is triggered by the leader (L) protein, devoid of any enzymatic activities. Here, we present evidence that expression of L alone was sufficient to cause permeabilization of the nuclear envelope in HeLa cells. In contrast to poliovirus, mengovirus infection of these cells did not elicit loss of nucleoporins Nup62 and Nup153 from the nuclear pore complex. Instead, nuclear envelope permeabilization was accompanied by hyperphosphorylation of Nup62 in cells infected with wild-type mengovirus, whereas both of these alterations were suppressed in L-deficient virus mutants. Since phosphorylation of Nup62 (although less prominent) did accompany permeabilization of the nuclear envelope prior to its mitotic disassembly in uninfected cells, we hypothesize that cardiovirus L alters the nucleocytoplasmic traffic by hijacking some components of the normal cell division machinery. The variability and biological significance of picornaviral interactions with the nucleocytoplasmic transport in the infected cells are discussed.

The experimental chemotherapeutic N6-furfuryladenosine (kinetin-riboside) induces rapid ATP depletion, genotoxic stress, and CDKN1A(p21) upregulation in human cancer cell lines

Cytokinins and cytokinin nucleosides are purine derivatives with potential anticancer activity. N(6)-furfuryladenosine (FAdo, kinetin-riboside) displays anti-proliferative and apoptogenic activity against various human cancer cell lines, and FAdo has recently been shown to suppress tumor growth in murine xenograft models of human leukemia and melanoma. In this study, FAdo-induced genotoxicity, stress response gene expression, and cellular ATP depletion were examined as early molecular consequences of FAdo exposure in MiaPaCa-2 pancreas carcinoma, A375 melanoma, and other human cancer cell lines. FAdo, but not adenosine or N(6)-furfuryladenine (FA), displayed potent anti-proliferative activity that was also observed in human primary fibroblasts and keratinocytes. Remarkably, massive ATP depletion and induction of genotoxic stress as assessed by the alkaline comet assay occurred within 60-180min of exposure to low micromolar concentrations of FAdo. This was followed by rapid upregulation of CDKN1A and other DNA damage/stress response genes (HMOX1, DDIT3, and GADD45A) as revealed by expression array and Western analysis. Pharmacological and siRNA-based genetic inhibition of adenosine kinase (ADK) suppressed FAdo cytotoxicity and also prevented ATP depletion and p21 upregulation suggesting the importance of bioconversion of FAdo into the nucleotide form required for drug action. Taken together our data suggest that early induction of genotoxicity and energy crisis are important causative factors involved in FAdo cytotoxicity.

Molecular dynamics, density functional, ADMET predictions, virtual screening, and molecular interaction field studies for identification and evaluation of novel potential CDK2 inhibitors in cancer therapy

In this work, we have used molecular dynamics, density functional theory, virtual screening, ADMET predictions, and molecular interaction field studies to design and propose eight novel potential inhibitors of CDK2. The eight molecules proposed showed interesting structural characteristics that are required for inhibiting the CDK2 activity and show potential as drug candidates for the treatment of cancer. The parameters related to the Rule of Five were calculated, and only one of the molecules violated more than one parameter. One of the proposals and one of the drug-like compounds selected by virtual screening indicated to be promising candidates for CDK2-based cancer therapy.

Involvement of septal Cdk5 in the emergence of excessive anxiety induced by stress

The aim of the present study was to evaluate whether the activation of Cdk5, a protein that has been suggested to participate in higher cognitive functions, is required for the onset of a sensitized anxiety-related behavior induced by stress. The exposure to restraint enhanced both Cdk5 expression in certain subareas of the septohippocampal system, principally in the lateral septum (LS) and septal Cdk5 kinase activity in rats. Behaviorally, restrained wild type mice showed a behavior indicative of enhanced anxiety in the elevated plus maze (EPM). In contrast, unstressed mice and stressed knockout mice, which lacked the p35 protein, the natural activator of Cdk5, displayed similar anxiety-like behavior in the EPM. Finally, the intra-LS infusion of olomoucine - a Cdk5 inhibitor - blocked the enhanced anxiety in the EPM induced by prior stress in rats. All these data provide evidence that septal Cdk5 is required in the emergence of a sensitized emotional process induced by stress.

Intracellular signaling pathways involved in post-mitotic dopaminergic PC12 cell death induced by 6-hydroxydopamine

Oxidative stress has been shown to mediate neuron damage in Parkinson's disease (PD). In the present report, we intend to clarify the intracellular pathways mediating dopaminergic neuron death after oxidative stress production using post-mitotic PC12 cells treated with the neurotoxin 6-hydroxydopamine (6-OHDA). The use of post-mitotic cells is crucial, because one of the suggested intracellular pathways implicated in neuron death relates to the re-entry of neurons (post-mitotic cells) in the cell cycle. We find that 6-OHDA sequentially increases intracellular oxidants, functional cell damage and caspase-3 activation, leading to cell death after 12 h of incubation. Prevention of cell damage by different antioxidants supports the implication of oxidative stress in the observed neurotoxicity. Oxidative stress-dependent phosphorylation of the MAPK JNK and oxidative stress-independent PKB/Akt dephosphorylation are involved in 6-OHDA neurotoxicity. Decrease in p21(WAF1/CIP1) and cyclin-D1 expression, disappearance of the non-phosphorylated band of retinoblastoma protein (pRb), and expression of proliferating cell nuclear antigen, not present in PC12 post-mitotic cells, suggest a re-entry of differentiated cells into cell cycle. Our results indicate that such a re-entry is mediated by oxidative stress and is involved in 6-OHDA-induced cell death. We conclude that at least three intracellular pathways are involved in 6-OHDA-induced cell death in differentiated PC12 cells: JNK activation, cell cycle progression (both oxidative stress-dependent), and Akt dephosphorylation (not related to the increase of oxidants); the three pathways are necessary for the cells to die, since blocking one of them is sufficient to keep the cells alive.

Pulse inhibition of histone deacetylases induces complete resistance to oxidative death in cortical neurons without toxicity and reveals a role for cytoplasmic p21(waf1/cip1) in cell cycle-independent neuroprotection

Histone deacetylase (HDAC) inhibitors are currently in human clinical trials as antitumor drugs because of their ability to induce cell dysfunction and death in cancer cells. The toxic effects of HDAC inhibitors are also apparent in cortical neurons in vitro, despite the ability of these agents to induce significant protection in the cells they do not kill. Here we demonstrate that pulse exposure of cortical neurons (2 h) in an in vitro model of oxidative stress results in durable neuroprotection without toxicity. Protection was associated with transcriptional upregulation of the cell cycle inhibitor, p21(waf1/cip1), both in this model and in an in vivo model of permanent ischemia. Transgenic overexpression of p21(waf1/cip1) in neurons can mimic the protective effect of HDAC inhibitors against oxidative stress-induced toxicity, including death induced by glutathione depletion or peroxide addition. The protective effect of p21(waf1/cip1) in the context of oxidative stress appears to be unrelated to its ability to act in the nucleus to inhibit cell cycle progression. However, although p21(waf1/cip1) is sufficient for neuroprotection, it is not necessary for HDAC inhibitor neuroprotection, because these agents can completely protect neurons cultured from p21(waf1/cip1)-null mice. Together these findings demonstrate (1) that pulse inhibition of HDACs in cortical neurons can induce neuroprotection without apparent toxicity; (2) that p21(waf1/cip1) is sufficient but not necessary to mimic the protective effects of HDAC inhibition; and (3) that oxidative stress in this model induces neuronal cell death via cell cycle-independent pathways that can be inhibited by a cytosolic, noncanonical action of p21(waf1/cip1).

The separation of antagonist from agonist effects of trisubstituted purines on CaV2.2 (N-type) channels

Dihydropyridines can affect L-type calcium channels (CaV1) as either agonists or antagonists. Seliciclib or R-roscovitine, a 2,6,9-trisubstituted purine, is a potent cyclin-dependent kinase inhibitor that induces both agonist and antagonist effects on CaV2 channels (N-, P/Q- and R-type). We studied the effects induced by various trisubstituted purines on CaV2.2 (N-type) channels to learn about chemical structure-function relationships. We found that S-roscovitine and R-roscovitine showed similar potency to inhibit, but agonist activity of S-roscovitine required at least a 20-fold higher concentration, suggesting stereospecificity of the agonist-binding site. The testing of other trisubstituted purines showed a correlation between CaV2.2 inhibition and cyclin-dependent kinase affinity that broke down after determining that a chemically unrelated inhibitor, kenpaullone, was a poor CaV2.2 inhibitor, and a kinase inactive analog (dimethylamino-olomoucine; DMAO) was a strong inhibitor, which together support a kinase independent effect. In fact, like dihydropyridine-induced L-channel inhibition, R-roscovitine left-shifted the closed-state inactivation versus voltage relationship, which suggests that inhibition results from CaV2 channels moving into the inactivated state. Trisubstituted purine antagonists could become clinically important drugs to treat diseases, such as heart failure and neuropathic pain that result from elevated CaV2 channel activity.

Maize D4;1 and D5 cyclin proteins in germinating maize. Associated kinase activity and regulation by phytohormones

We have previously reported the expression of four different maize D cyclins during seed germination and showed that cytokinins and auxins stimulate the expression of every cyclin in a differential way. In this paper we characterize the behavior at the protein level of two of these cyclins, CycD5 and CycD4;1. Antibodies were raised against CycD5;2 (which very likely also recognizes D5;1) and CycD4;1 and Western blot studies demonstrated that neither BA nor indol-3 acetic acid (IAA) stimulate cyclin accumulation during germination, compared with control levels. However, phytohormones, particularly IAA, modify the kinase activity associated to D cyclins preferentially at early hours of germination. The associated kinase moiety to D cyclins appears to be of a Cdk-A type because this protein immunoprecipitates with D cyclins and because kinase activity is strongly inhibited by both olomoucine and also by a peptide corresponding to the carboxy end of a maize kip related protein (KRP) protein. There is thus no correlation between mRNA and protein expression for these maize D cyclins during seed germination, although phytohormones may stimulate a signaling cascade that stimulates activation of protein kinase activity in cyclin-Cdk complexes.

CDK/GSK-3 inhibitors as therapeutic agents for parenchymal renal diseases

Drug discovery to lessen the burden of chronic renal failure and end-stage renal disease remains a principle goal of translational research in nephrology. In this review, we provide an overview of the current development of small molecule cyclin-dependent kinase (CDK)/glycogen synthase kinase-3 (GSK-3) inhibitors as therapeutic agents for parenchymal renal diseases. The emergence of this drug family has resulted from the recognition that CDKs and GSK-3s play critical roles in the progression and regression of many kidney diseases. CDK/GSK-3 inhibitors suppress pathogenic proliferation, apoptosis, and inflammation, and promote regeneration of injured tissue. Preclinical efficacy has now been demonstrated in mesangial proliferative glomerulonephritis, crescentic glomerulonephritis, collapsing glomerulopathy, proliferative lupus nephritis, polycystic kidney diseases, diabetic nephropathy, and several forms of acute kidney injury. Novel biomarkers of therapy are aiding the process of drug development. This review will highlight these advancements in renal therapeutics.

Untangling tau hyperphosphorylation in drug design for neurodegenerative diseases

Aggregation of hyperphosphorylated tau is one of the characteristic neuropathological lesions of Alzheimer's disease and other neurodegenerative disorders. Pharmacological modulation of tau hyperphosphorylation might represent a valid and feasible therapeutic strategy for such disorders. Here, we consider recent evidence supporting the validity of the three most relevant kinases affecting tau hyperphosphorylation - GSK3beta, CDK5 and ERK2 - as drug targets and describe progress in the design of inhibitors for these kinases.

In silico three-dimensional pharmacophores for aiding the discovery of the Pfmrk (Plasmodium cyclin-dependent protein kinases) specific inhibitors for the therapeutic treatment of malaria

The resurgence of malaria and lack of effective antimalarial drugs affect millions of people worldwide every year, causing several million deaths. With the emergence of structure-based drug design methodologies, a major thrust in drug discovery efforts has shifted towards targeting specific proteins in parasites that are involved in their metabolic pathways. Although cyclin-dependent kinases (CDKs), due to their direct role in cell cycle regulations, have been targeted for the development of cancer therapeutics, CDKs for Plasmodium falciparum have only been recently identified to be attractive for the discovery of antimalarials. One of the plasmodium CDK targets is Pfmrk. Being a putative homolog of Cdk7 and, thus, having the possibility of dual functions, both in cell cycle control and gene expression within the parasite, pfrmk has become an interesting antimalarial chemotherapeutic target. This review discusses how in silico methodologies, without the knowledge of the X-ray crystallographic structure of Pfmrk, particularly based on the development of pharmacophores on known inhibitors can aid the discovery and design of Pfmrk-specific inhibitors through virtual screening of compound databases and provides insights into the understanding of the mechanism of binding in the active site of this enzyme.

Cyclin-dependent kinase inhibitors block erythrocyte invasion and intraerythrocytic development of Babesia bovis in vitro

Cyclin-dependent kinases (CDKs) are essential for the regulation of the eukaryotic cell cycle. A number of chemicals, which selectively inhibit the CDK activities, have been synthesized for the development of anti-cancer drugs. This report describes the inhibitory effect of purine derivatives known to be CDK inhibitors on the asexual growth of Babesia bovis. The 4 compounds, roscovitine, purvalanol A, CGP74514A, and CDK2 Inhibitor II, showed significantly suppressive effects on the in vitro growth of B. bovis. Three (roscovitine, purvalanol A, and CDK2 Inhibitor II) showed an inhibitory effect on the early stages of intraerythrocytic development of B. bovis. CGP74514A (CDK1-specific inhibitor) blocked the erythrocyte invasion by merozoites. Our data suggest the chemotherapeutic potential of the CDK inhibitors for babesiosis, and the target molecules of the compounds would participate in the process of successful erythrocyte invasion or intraerythrocytic development of B. bovis.

Classical Anticytokinins Do Not Interact with Cytokinin Receptors but Inhibit Cyclin-dependent Kinases

Cytokinins are a class of plant hormones that regulate the cell cycle and diverse developmental and physiological processes. Several compounds have been identified that antagonize the effects of cytokinins. Based on structural similarities and competitive inhibition, it has been assumed that these anticytokinins act through a common cellular target, namely the cytokinin receptor. Here, we examined directly the possibility that various representative classical anticytokinins inhibit the Arabidopsis cytokinin receptors CRE1/AHK4 (cytokinin response 1/Arabidopsis histidine kinase 4) and AHK3 (Arabidopsis histidine kinase 3). We show that pyrrolo[2,3-d]pyrimidine and pyrazolo[4,3-d]pyrimidine anticytokinins do not act as competitors of cytokinins at the receptor level. Flow cytometry and microscopic analyses revealed that anticytokinins inhibit the cell cycle and cause disorganization of the microtubular cytoskeleton and apoptosis. This is consistent with the hypothesis that they inhibit regulatory cyclin-dependent kinase (CDK) enzymes. Biochemical studies demonstrated inhibition by selected anti-cytokinins of both Arabidopsis and human CDKs. X-ray determination of the crystal structure of a human CDK2-anticytokinin complex demonstrated that the antagonist occupies the ATP-binding site of CDK2. Finally, treatment of human cancer cell lines with anticytokinins demonstrated their ability to kill human cells with similar effectiveness as known CDK inhibitors.

Galectin-3 may contribute to Cisplatin resistance in clear cell carcinoma of the ovary

Our previous findings suggested that lower cell proliferation of clear cell carcinoma (CCC) of the ovary may contribute to its resistance to chemotherapy. We conducted the present study to find the gene that regulates cell proliferation of CCC and to elucidate whether it contributes to cisplatin (CDDP) resistance. Complementary DNA microarray analysis revealed that the gene expression level of galectin-3 of CCC cell lines (KK, RMG-I, HAC-2) was over threefold higher than that of ovarian serous adenocarcinoma (SAC) cell lines (HRA, KF). S-phase fraction increased after knocking down galectin-3 using small interfering RNA in RMG-I, KK, and HAC-2 cells. The protein expression of p27 decreased after knocking down galectin-3. CDDP-induced apoptosis was increased after knocking down galectin-3, and this cytotoxic effect was canceled by roscovitine. Immunohistochemical staining showed that galectin-3 expression in tumors of 20 CCC was significantly more frequent than that of 20 SAC (70.0% vs 15.0%, P = 0.0004). The present study showed that the expression of galectin-3 in CCC might contribute to its lower cell proliferation and lead to CDDP resistance.

Five years of progress on cyclin-dependent kinases and other cellular proteins as potential targets for antiviral drugs

In 1997-1998, the pharmacological cyclin-dependent kinase (CDK) inhibitors (PCIs) were independently discovered to inhibit replication of human cytomegalovirus, herpes simplex virus type 1 and HIV-1. The results from small clinical trials against cancer were then suggesting that PCIs could be safe enough to be used clinically. It was thus hypothesized that PCIs could have the potential to be developed as novel antivirals targeting cellular proteins. Consequently, Antiviral Chemistry & Chemotherapy published in 2001 the first review on the potential of CDKs, and cellular proteins in general, as potential targets for antivirals. The viral functions inhibited by PCIs, or their cellular targets, were then just starting to be characterized. The antiviral spectrum of PCIs and their effects on viral disease were still mostly untested. Even their actual specificity was not yet completely characterized. In addition, cellular proteins were not accepted as valid targets for antivirals. Significant progress has been made in the last 5 years in understanding the antiviral activities of PCIs and the potential roles of cellular proteins in general as targets for antivirals. The first clinical trials of the antiviral activities of PCIs and other inhibitors of cellular protein kinases have now been scheduled. Herein, we review the progress made since the publication of the first review on PCIs as potential antiviral drugs and on CDKs, and cellular proteins in general, as potential targets for antiviral drugs. We also highlight the major issues that still need to be addressed before PCIs or other drugs targeting cellular proteins can be developed as clinical antivirals.

Preparation, biological activity and endogenous occurrence of N6-benzyladenosines

Cytokinin activity of forty-eight 6-benzyladenosine derivatives at both the receptor and cellular levels as well as their anticancer properties were compared in various in vitro assays. The compounds were prepared by the condensation of 6-chloropurine riboside with corresponding substituted benzylamines and characterized by standard collection of physico-chemical methods. The majority of synthesized derivatives exhibited high activity in all three of the cytokinin bioassays used (tobacco callus, wheat leaf senescence and Amaranthus bioassay). The highest activities were observed in the senescence bioassay. For several of the compounds tested, significant differences in activity were found between the bioassays used, indicating that diverse recognition systems may operate. This suggests that it may be possible to modulate particular cytokinin-dependent processes with specific compounds. In contrast to their high activity in bioassays, the tested compounds were recognized with only very low sensitivity in both Arabidopsis thaliana AHK3 and AHK4 receptor assays. The prepared derivatives were also investigated for their antiproliferative properties on cancer and normal cell lines. Several of them showed very strong cytotoxic activity against various cancer cell lines. On the other hand, they were not cytotoxic for normal murine fibroblast (NIH/3T3) cell line. This anticancer activity of cytokinin ribosides may be important, given that several of them occur as endogenous compounds in different organisms.

Phosphorylation of Protein Phosphatase 1 by Cyclin-dependent Protein Kinase 5 during Nerve Growth Factor-induced PC12 Cell Differentiation

The transcription factor Egr-1 activates cyclin-dependent protein kinase 5 (Cdk5) during nerve growth factor (NGF)-induced differentiation of PC12 cells into neurons (Harada, T. Morooka, T., Ogawa, S., and Nishida, E. (2001) Nat. Cell Biol. 3, 453-459). The downstream target of Cdk5 in the Egr-1/Cdk5 pathway is not clear. In this study, we observed that phosphorylation of protein phosphatase 1 (PP1) on Thr(320) is reduced in brain extracts from Egr-1(-/-) mice, indicating that a kinase downstream of Egr-1 phosphorylates PP1. In HEK 293 cells co-transfected with PP1 and Cdk5, Cdk5 phosphorylates PP1. In vitro, Cdk5 purified from bovine brain phosphorylates bacterially expressed recombinant PP1. In NGF-treated PC12 cells, inhibition of Cdk5 by olomoucine or silencing Cdk5 expression by small interfering RNA strategy, suppresses PP1 phosphorylation. Silencing Cdk5 expression by small interfering RNA also blocks NGF-induced neurite outgrowth. Overexpression of PP1 (wild type) promotes NGF-induced differentiation of PC12 cells, whereas that of PP1 (T320A) has no effect. Our data indicate that PP1 is a downstream target of the NGF/Egr-1/Cdk5 pathway during NGF-induced differentiation of PC12 cells and suggest that PP1 phosphorylation promotes neuronal differentiation.