Mixed lineage kinase activity of indolocarbazole analogues

Kinases control of regulated cell death revealing druggable targets for Parkinson's disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative disorder in the world. Motor impairment seen in PD is associated with dopaminergic neurotoxicity in the striatum, and dopaminergic neuronal death in the substantia nigra pars compacta. Cell death has a significant effect on the development and progression of PD. Extensive research over the last few decades has unveiled new regulated cell death (RCD) mechanisms that are not dependent on apoptosis such as necroptosis, ferroptosis, and others. In this review, we will overview the mechanistic pathways of different types of RCD. Unlike accidental cell death, RCD subroutines can be regulated and the RCD-associated kinases are potential druggable targets. Hence, we will address an overview and analysis of different kinases regulating apoptosis such as receptor-interacting protein kinase 1 (RIPK-1), RIPK3, mixed lineage kinase (MLK), Ataxia telangiectasia muted (ATM), cyclin-dependent kinase (CDK), death-associated protein kinase 1 (DAPK1), Apoptosis-signaling kinase-1 (ASK-1), and Leucine-rich repeat kinase-2 (LRRK2). In addition to the role of RIPK1, RIPK3, and Mixed Lineage Kinase Domain like Pseudokinase (MLKL) in necroptosis. We also overview functions of AMP-kinase (AMPK), protein kinase C (PKC), RIPK3, and ATM in ferroptosis. We will recap the anti-apoptotic, anti-necroptotic, and anti-ferroptotic effects of different kinase inhibitors in different models of PD. Finally, we will discuss future challenges in the repositioning of kinase inhibitors in PD. In conclusion, this review kicks-start targeting RCD from a kinases perspective, opening novel therapeutic disease-modifying therapeutic avenues for PD.

Phosphorylation of mixed lineage kinase MLK3 by cyclin-dependent kinases CDK1 and CDK2 controls ovarian cancer cell division

Mixed lineage kinase 3 (MLK3) is a serine/threonine mitogen-activated protein kinase kinase kinase that promotes the activation of multiple mitogen-activated protein kinase pathways and is required for invasion and proliferation of ovarian cancer cells. Inhibition of MLK activity causes G2/M arrest in HeLa cells; however, the regulation of MLK3 during ovarian cancer cell cycle progression is not known. Here, we found that MLK3 is phosphorylated in mitosis and that inhibition of cyclin-dependent kinase 1 (CDK1) prevented MLK3 phosphorylation. In addition, we observed that c-Jun N-terminal kinase, a downstream target of MLK3 and a direct target of MKK4 (SEK1), was activated in G2 phase when CDK2 activity is increased and then inactivated at the beginning of mitosis concurrent with the increase in CDK1 and MLK3 phosphorylation. Using in vitro kinase assays and phosphomutants, we determined that CDK1 phosphorylates MLK3 on Ser548 and decreases MLK3 activity during mitosis, whereas CDK2 phosphorylates MLK3 on Ser770 and increases MLK3 activity during G1/S and G2 phases. We also found that MLK3 inhibition causes a reduction in cell proliferation and a cell cycle arrest in ovarian cancer cells, suggesting that MLK3 is required for ovarian cancer cell cycle progression. Taken together, our results suggest that phosphorylation of MLK3 by CDK1 and CDK2 is important for the regulation of MLK3 and c-Jun N-terminal kinase activities during G1/S, G2, and M phases in ovarian cancer cell division.

Therapeutic potential of targeting mixed lineage kinases in cancer and inflammation

Dysregulation of intracellular signaling pathways is a key attribute of diseases associated with chronic inflammation, including cancer. Mitogen activated protein kinases have emerged as critical conduits of intracellular signal transmission, yet due to their ubiquitous roles in cellular processes, their direct inhibition may lead to undesired effects, thus limiting their usefulness as therapeutic targets. Mixed lineage kinases (MLKs) are mitogen-activated protein kinase kinase kinases (MAP3Ks) that interact with scaffolding proteins and function upstream of p38, JNK, ERK, and NF-kappaB to mediate diverse cellular signals. Studies involving gene silencing, genetically engineered mouse models, and small molecule inhibitors suggest that MLKs are critical in tumor progression as well as in inflammatory processes. Recent advances indicate that they may be useful targets in some types of cancer and in diseases driven by chronic inflammation including neurodegenerative diseases and metabolic diseases such as nonalcoholic steatohepatitis. This review describes existing MLK inhibitors, the roles of MLKs in various aspects of tumor progression and in the control of inflammatory processes, and the potential for therapeutic targeting of MLKs.

Emerging therapies for Parkinson's disease: from bench to bedside

The prevalence of Parkinson's disease (PD) increases with age and is projected to increase in parallel to the rising average age of the population. The disease can have significant health-related, social, and financial implications not only for the patient and the caregiver, but for the health care system as well. While the neuropathology of this neurodegenerative disorder is fairly well understood, its etiology remains a mystery, making it difficult to target therapy. The currently available drugs for treatment provide only symptomatic relief and do not control or prevent disease progression, and as a result patient compliance and satisfaction are low. Several emerging pharmacotherapies for PD are in different stages of clinical development. These therapies include adenosine A2A receptor antagonists, glutamate receptor antagonists, monoamine oxidase inhibitors, anti-apoptotic agents, and antioxidants such as coenzyme Q10, N-acetyl cysteine, and edaravone. Other emerging non-pharmacotherapies include viral vector gene therapy, microRNAs, transglutaminases, RTP801, stem cells and glial derived neurotrophic factor (GDNF). In addition, surgical procedures including deep brain stimulation, pallidotomy, thalamotomy and gamma knife surgery have emerged as alternative interventions for advanced PD patients who have completely utilized standard treatments and still suffer from persistent motor fluctuations. While several of these therapies hold much promise in delaying the onset of the disease and slowing its progression, more pharmacotherapies and surgical interventions need to be investigated in different stages of PD. It is hoped that these emerging therapies and surgical procedures will strengthen our clinical armamentarium for improved treatment of PD.

Inhibition of fast axonal transport by pathogenic SOD1 involves activation of p38 MAP kinase

Dying-back degeneration of motor neuron axons represents an established feature of familial amyotrophic lateral sclerosis (FALS) associated with superoxide dismutase 1 (SOD1) mutations, but axon-autonomous effects of pathogenic SOD1 remained undefined. Characteristics of motor neurons affected in FALS include abnormal kinase activation, aberrant neurofilament phosphorylation, and fast axonal transport (FAT) deficits, but functional relationships among these pathogenic events were unclear. Experiments in isolated squid axoplasm reveal that FALS-related SOD1 mutant polypeptides inhibit FAT through a mechanism involving a p38 mitogen activated protein kinase pathway. Mutant SOD1 activated neuronal p38 in mouse spinal cord, neuroblastoma cells and squid axoplasm. Active p38 MAP kinase phosphorylated kinesin-1, and this phosphorylation event inhibited kinesin-1. Finally, vesicle motility assays revealed previously unrecognized, isoform-specific effects of p38 on FAT. Axon-autonomous activation of the p38 pathway represents a novel gain of toxic function for FALS-linked SOD1 proteins consistent with the dying-back pattern of neurodegeneration characteristic of ALS.

Loss of MLK3 signaling impedes ulcer healing by modulating MAPK signaling in mouse intestinal mucosa

Mixed-lineage kinase 3 (MLK3) activates multiple MAPK pathways and can initiate apoptosis, proliferation, migration, or differentiation in different cell types. However, whether MLK3 signaling regulates intestinal epithelial cell sheet migration in vivo is not known. We sought to investigate whether MLK3 signaling is important in intestinal mucosal healing and epithelial cell motility in vivo and in vitro. In vivo, we compared the healing of jejunal mucosal ulcers induced in MLK3 knockout (KO) mice with healing in wild-type (WT) mice. Ulcer healing was 20.8% less at day 3 (P < 0.05) and 18.9% less at day 5 (P < 0.05) in MLK3 KO than WT mice. Within the intestinal mucosa of MLK3 KO mice, ERK and JNK signaling were reduced, phosphatase and tensin homolog deleted on chromosome 10 (PTEN) level was increased, and p38 signaling was unchanged. Parallel in vitro studies using an MLK inhibitor assessed the role of MLK signaling in human Caco-2 intestinal epithelial migration across collagen substrates. The MLK inhibitor reduced closure of circular wounds in Caco-2 monolayers. MLK inhibition reduced ERK and JNK, but not p38, signaling in Caco-2 cells. Although PTEN is increased after MLK inhibition, it does not influence MLK-mediated cell migration. These findings indicate that disruption of MLK3 signaling impairs ulcer healing by suppressing ERK and JNK signaling in vitro and in mouse intestinal mucosa in vivo. These results reveal a novel role for MLK3 signaling in the regulation of intestinal epithelial migration in vivo and suggest that MLK3 may be an important target for the regulation of intestinal mucosal healing.

AAK1 identified as an inhibitor of neuregulin-1/ErbB4-dependent neurotrophic factor signaling using integrative chemical genomics and proteomics

Target identification remains challenging for the field of chemical biology. We describe an integrative chemical genomic and proteomic approach combining the use of differentially active analogs of small molecule probes with stable isotope labeling by amino acids in cell culture-mediated affinity enrichment, followed by subsequent testing of candidate targets using RNA interference-mediated gene silencing. We applied this approach to characterizing the natural product K252a and its ability to potentiate neuregulin-1 (Nrg1)/ErbB4 (v-erb-a erythroblastic leukemia viral oncogene homolog 4)-dependent neurotrophic factor signaling and neuritogenesis. We show that AAK1 (adaptor-associated kinase 1) is a relevant target of K252a, and that the loss of AAK1 alters ErbB4 trafficking and expression levels, providing evidence for a previously unrecognized role for AAK1 in Nrg1-mediated neurotrophic factor signaling. Similar strategies should lead to the discovery of novel targets for therapeutic development.

ZPK/DLK, a mitogen-activated protein kinase kinase kinase, is a critical mediator of programmed cell death of motoneurons

Activation of mitogen-activated protein kinase pathways is critically involved in naturally occurring programmed cell death of motoneurons during development, but the upstream mediators remain undetermined. We found that mice deficient in ZPK, also called DLK (ZPK/DLK), an upstream kinase in these pathways, have twice as many spinal motoneurons as do their wild-type littermates. Nuclear HB9/MNX1-positive motoneuron pools were generated similarly in the spinal cord of both ZPK/DLK-deficient and wild-type embryos. Thereafter, however, significantly less apoptotic motoneurons were found in ZPK/DLK-deficient embryos compared with wild-type embryos, resulting in retention of excess numbers of motoneurons after birth. Notably, these excess motoneurons remained viable without atrophic changes in the ZPK/DLK-deficient mice surviving into adulthood. Analysis of the diaphragm and the phrenic nerve revealed that clustering and innervation of neuromuscular junctions were indistinguishable between ZPK/DLK-deficient and wild-type mice, whereas the proximal portion of the phrenic nerve of ZPK/DLK-deficient mice contained significantly more axons than the distal portion. This result supports the hypothesis that some excess ZPK/DLK-deficient motoneurons survived without atrophy despite failure to establish axonal contact with their targets. This study provides compelling evidence for a critical role for ZPK/DLK in naturally occurring programmed cell death of motoneurons and suggests that ZPK/DLK could become a strategic therapeutic target in motor neuron diseases in which aberrant activation of the apoptogenic cascade is involved.

MLK3 is critical for breast cancer cell migration and promotes a malignant phenotype in mammary epithelial cells

The malignant phenotype in breast cancer is driven by aberrant signal transduction pathways. Mixed-lineage kinase-3 (MLK3) is a mammalian mitogen-activated protein kinase kinase kinase (MAP3K) that activates multiple MAPK pathways. Depending on the cellular context, MLK3 has been implicated in apoptosis, proliferation, migration and differentiation. Here we investigated the effect of MLK3 and its signaling to MAPKs in the acquisition of malignancy in breast cancer. We show that MLK3 is highly expressed in breast cancer cells. We provide evidence that MLK3's catalytic activity and signaling to c-jun N-terminal kinase (JNK) is required for migration of highly invasive breast cancer cells and for MLK3-induced migration of mammary epithelial cells. Expression of active MLK3 is sufficient to induce the invasion of mammary epithelial cells, which requires AP-1 activity and is accompanied by the expression of several proteins corresponding to AP-1-regulated invasion genes. To assess MLK3's contribution to the breast cancer malignant phenotype in a more physiological setting, we implemented a strategy to inducibly express active MLK3 in the preformed acini of MCF10A cells grown in 3D Matrigel. Induction of MLK3 expression dramatically increases acinar size and modestly perturbs apicobasal polarity. Remarkably, MLK3 expression induces luminal repopulation and suppresses the expression of the pro-apoptotic protein BimEL, as has been observed in Her2/Neu-expressing acini. Taken together, our data show that MLK3-JNK-AP-1 signaling is critical for breast cancer cell migration and invasion. Our current study uncovers both a proliferative and novel antiapoptotic role for MLK3 in the acquisition of a malignant phenotype in mammary epithelial cells. Thus, MLK3 may be an important therapeutic target for the treatment of invasive breast cancer.

Mixed lineage kinase-3 stabilizes and functionally cooperates with TRIBBLES-3 to compromise mitochondrial integrity in cytokine-induced death of pancreatic beta cells

Mixed lineage kinases (MLKs) have been implicated in cytokine signaling as well as in cell death pathways. Our studies show that MLK3 is activated in leukocyte-infiltrated islets of non-obese diabetic mice and that MLK3 activation compromises mitochondrial integrity and induces apoptosis of beta cells. Using an ex vivo model of islet-splenocyte co-culture, we show that MLK3 mediates its effects via the pseudokinase TRB3, a mammalian homolog of Drosophila Tribbles. TRB3 expression strongly coincided with conformational change and mitochondrial translocation of BAX. Mechanistically, MLK3 directly interacted with and stabilized TRB3, resulting in inhibition of Akt, a strong suppressor of BAX translocation and mitochondrial membrane permeabilization. Accordingly, attenuation of MLK3 or TRB3 expression each prevented cytokine-induced BAX conformational change and attenuated the progression to apoptosis. We conclude that MLKs compromise mitochondrial integrity and suppress cellular survival mechanisms via TRB3-dependent inhibition of Akt.

The mixed-lineage kinase 1-3 signalling pathway regulates stress response in cardiac myocytes via GATA-4 and AP-1 transcription factors

BACKGROUND AND PURPOSE

The mixed-lineage kinases (MLKs) act upstream of mitogen-activated protein kinases, but their role in cardiac biology and pathology is largely unknown.

EXPERIMENTAL APPROACH

We investigated the effect of a MLK1-3 inhibitor CEP-11004 on G protein-coupled receptor agonist-induced stress response in neonatal rat cardiac myocytes in culture.

KEY RESULTS

CEP-11004 administration dose-dependently attenuated phenylephrine and endothelin-1 (ET-1)-induced c-Jun N-terminal kinase activation. MLK inhibition also reduced ET-1- and phenylephrine-induced phosphorylation of p38 mitogen-activated protein kinase. In contrast, phenylephrine-induced extracellular signal-regulated kinase phosphorylation was further up-regulated by CEP-11004. ET-1 increased activator protein-1 binding activity 3.5-fold and GATA-binding protein 4 (GATA-4) binding activity 1.8-fold, both of which were attenuated with CEP-11004 administration by 59% and 63% respectively. Phenylephrine induced activator protein-1 binding activity by 2.6-fold, which was decreased by 81% with CEP-11004 administration. Phenylephrine also induced a 3.7-fold increase in the transcriptional activity of B-type natriuretic peptide (BNP), which was attenuated by 41% with CEP-11004 administration. In agreement, MLK inhibition also reduced hypertrophic agonist-induced secretion of immunoreactive atrial natriuretic peptide and BNP.

CONCLUSIONS AND IMPLICATIONS

These results showed that inhibition of the MLK1-3 signalling pathway was sufficient for suppressing the activity of key nuclear effectors (GATA-4 and activator protein-1 transcription factors) in cardiac hypertrophy, and attenuated the agonist-induced atrial natriuretic peptide secretion and activation of BNP gene transcription.

Fates of neurotrophins after retrograde axonal transport: phosphorylation of p75NTR is a sorting signal for delayed degradation

Neurotrophins can mediate survival or death of neurons. Opposing functions of neurotrophins are based on binding of these ligands to two distinct types of receptors: trk receptors and p75NTR. Previous work showed that target-derived NGF induces cell death, whereas BDNF and NT-3 enhance survival of neurons in the isthmo-optic nucleus of avian embryos. To determine the fate of retrogradely transported neurotrophins and test whether their sorting differs between neurotrophins mediating survival- or death-signaling pathways, we traced receptor-binding, sorting, and degradation kinetics of target-applied radiolabeled neurotrophins that bind in this system to trk receptors (BDNF, NT-3) or only to p75NTR (NGF). At the ultrastructural level, the p75NTR-bound NGF accumulates with a significant delay in multivesicular bodies and organelles of the degradation pathway on arrival in the cell body when compared with trk-bound BDNF or NT-3. This delayed lysosomal accumulation was restricted to target-derived NGF, but was not seen when NGF was supplied to the soma in vitro. The kinase inhibitors K252a and Gö6976 alter the kinetics of organelle accumulation: phosphorylation of p75NTR is a sorting signal for delayed sequestering of p75NTR-bound NGF in multivesicular bodies and delayed degradation in lysosomes when compared with trk-bound neurotrophins. Mutagenesis and mass spectrometry studies indicate that p75NTR is phosphorylated by conventional protein kinase C on serine 266. We conclude that, in addition to the known phosphorylation of trks, the phosphorylation of p75NTR can also significantly affect neuronal survival in vivo by changing the intracellular sorting and degradation kinetics of its ligands and thus signaling duration.

GCK is essential to systemic inflammation and pattern recognition receptor signaling to JNK and p38

Systemic inflammation arising from the organismal distribution of pathogen-associated molecular patterns is a major cause of clinical morbidity and mortality. Herein we report a critical and previously unrecognized in vivo role for germinal center kinase (GCK, genome nomenclature: map4k2), a mammalian Sterile 20 (STE20) orthologue, in PAMP signaling, and systemic inflammation. We find that disruption of gck in mice strongly impairs PAMP-stimulated macrophage cytokine and chemokine release and renders mice resistant to endotoxin-mediated lethality. Bone marrow transplantation studies show that hematopoietic cell GCK signaling is essential to systemic inflammation. Disruption of gck substantially reduces PAMP activation of macrophage Jun-N-terminal kinase (JNK) and p38 mitogen-activated protein kinases (MAPKs) via reduced activation of the MAPK-kinase-kinases (MAP3Ks) mixed lineage kinases (MLKs)-2 and -3. Extracellular signal-regulated kinase (ERK) and nuclear factor-kappaB (NF-kappaB) activation are largely unaffected. Thus, GCK is an essential PAMP effector coupling JNK and p38, but not ERK or NF-kappaB to systemic inflammation.

Opposing roles for ATF2 and c-Fos in c-Jun-mediated neuronal apoptosis

The activator protein 1 (AP-1) transcription factor c-Jun is crucial for neuronal apoptosis. However, c-Jun dimerization partners and the regulation of these proteins in neuronal apoptosis remain unknown. Here we report that c-Jun-mediated neuronal apoptosis requires the concomitant activation of activating transcription factor-2 (ATF2) and downregulation of c-Fos. Furthermore, we have observed that c-Jun predominantly heterodimerizes with ATF2 and that the c-Jun/ATF2 complex promotes apoptosis by triggering ATF activity. Inhibition of c-Jun/ATF2 heterodimerization using dominant negative mutants, small hairpin RNAs, or decoy oligonucleotides was able to rescue neurons from apoptosis, whereas constitutively active ATF2 and c-Jun mutants were found to synergistically stimulate apoptosis. Bimolecular fluorescence complementation analysis confirmed that, in living neurons, c-Fos downregulation facilitates c-Jun/ATF2 heterodimerization. A chromatin immunoprecipitation assay also revealed that c-Fos expression prevents the binding of c-Jun/ATF2 heterodimers to conserved ATF sites. Moreover, the presence of c-Fos is able to suppress the expression of c-Jun/ATF2-mediated target genes and, therefore, apoptosis. Taken together, our findings provide evidence that potassium deprivation-induced neuronal apoptosis is mediated by concurrent upregulation of c-Jun/ATF2 heterodimerization and downregulation of c-Fos expression. This paradigm demonstrates opposing roles for ATF2 and c-Fos in c-Jun-mediated neuronal apoptosis.

Mixed-lineage kinase 1 and mixed-lineage kinase 3 subtype-selective dihydronaphthyl[3,4-a]pyrrolo[3,4-c]carbazole-5-ones: optimization, mixed-lineage kinase 1 crystallography, and oral in vivo activity in 1-methyl-4-phenyltetrahydropyridine models

The optimization of the dihydronaphthyl[3,4-a]pyrrolo[3,4-c]carbazole-5-one R(2) and R(12) positions led to the identification of the first MLK1 and MLK3 subtype-selective inhibitors within the MLK family. Compounds 14 (CEP-5104) and 16 (CEP-6331) displayed good potency for MLK1 and MLK3 inhibition with a greater than 30- to 100-fold selectivity for related family members MLK2 and DLK. Compounds 14 and 16 were orally active in vivo in a mouse MPTP biochemical efficacy model that was comparable to the first-generation pan-MLK inhibitor 1 (CEP-1347). The MLK1 structure-activity relationships were supported by the first-reported X-ray crystal structure of MLK1 bound with 16.

CEP-1347 reduces mutant huntingtin-associated neurotoxicity and restores BDNF levels in R6/2 mice

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by an expanded polyglutamine repeat within the protein Huntingtin (Htt). We previously reported that mutant Htt expression activates the ERK1/2 and JNK pathways [Apostol, B.L., Illes, K., Pallos, J., Bodai, L., Wu, J., Strand, A., Schweitzer, E.S., Olson, J.M., Kazantsev, A., Marsh, J.L., Thompson, L.M., 2006. Mutant huntingtin alters MAPK signaling pathways in PC12 and striatal cells: ERK1/2 protects against mutant huntingtin-associated toxicity. Hum. Mol. Genet. 15, 273-285]. Chemical and genetic modulation of these pathways promotes cell survival and death, respectively. Here we test the ability of two closely related compounds, CEP-11004 and CEP-1347, which inhibit Mixed Lineage Kinases (MLKs) and are neuroprotective, to suppress mutant Htt-mediated pathogenesis in multiple model systems. CEP-11004/CEP-1347 treatment significantly decreased toxicity in mutant Htt-expressing cells that evoke a strong JNK response. However, suppression of cellular dysfunction in cell lines that exhibit only mild Htt-associated toxicity and little JNK activation was associated with activation of ERK1/2. These compounds also reduced neurotoxicity in immortalized striatal neurons from mutant knock-in mice and Drosophila expressing a mutant Htt fragment. Finally, CEP-1347 improved motor performance in R6/2 mice and restored expression of BDNF, a critical neurotrophic factor that is reduced in HD. These studies suggest a novel therapeutic approach for a currently untreatable neurodegenerative disease, HD, via CEP-1347 up-regulation of BDNF.

Antiapoptotic and trophic effects of dominant-negative forms of dual leucine zipper kinase in dopamine neurons of the substantia nigra in vivo

There is extensive evidence that the mitogen-activated protein kinase (MAPK) signaling cascade mediates programmed cell death in neurons. However, current evidence that the mixed linage kinases (MLKs), upstream in this cascade, mediate cell death is based, in the in vivo context, entirely on pharmacological approaches. The compounds used in these studies have neither complete specificity nor selectivity among these kinases. Therefore, to better address the molecular specificity of the MLKs in mediating neuron death, we used dominant-negative constructs delivered by AAV (adenoassociated virus) vector transfer. We assessed effects in a neurotoxin model of parkinsonism, in which neuroprotection by pharmacologic MLK inhibition has been reported. We find that two dominant-negative forms of dual leucine zipper kinase (DLK) inhibit apoptosis and enhance long-term survival of dopamine neurons, but a dominant negative of MLK3 does not. Interestingly, the kinase-dead form of DLK not only blocks apoptosis but also has trophic effects on dopamine neurons. Although the MAPK cascade activates a number of downstream cell death mediators, we find that inhibition of DLK correlates closely with blockade of phosphorylation of c-jun and prevention of cell death. We conclude that DLK acts primarily through c-jun phosphorylation to mediate cell death in this model.

Activating transcription factor 3 up-regulated by c-Jun NH(2)-terminal kinase/c-Jun contributes to apoptosis induced by potassium deprivation in cerebellar granule neurons

Cerebellar granule neurons (CGNs) depend on potassium depolarization for survival and undergo apoptosis when deprived of depolarizing concentration of potassium. Activating transcription factor 3 (ATF3), a stress-inducible protein, belongs to the ATF/CREB family of transcription factors family and is involved in cell growth and apoptosis. However, the role of ATF3 in neuronal apoptosis remains unknown. Here, we showed that ATF3 was up-regulated under potassium deprivation in CGNs, and this induction was preceded by a rapid and sustained activation of c-Jun NH(2)-terminal kinase/c-Jun signaling pathway, which plays a fundamental role in neuronal apoptosis. Furthermore, ATF3 up-regulation was abolished by inhibition of JNK or knockdown of c-Jun. Finally, knockdown of ATF3 by RNA interference protected CGNs from potassium deprivation-induced apoptosis. Taken together, our results indicate that ATF3 is a downstream target of JNK/c-Jun pathway and contributes to apoptosis induced by potassium deprivation in rat CGNs.

p38 and JNK MAPK, but not ERK1/2 MAPK, play important role in colchicine-induced cortical neurons apoptosis

Colchicine is a microtubule interfering agent and is able to induce neural apoptosis. However, the intracellular pathway involved in its neurotoxicity is still unclear. In the present study, three of mitogen-activated protein kinases (MAPKs): p38, c-Jun N-terminal kinase (JNK) and extracellular-regulated kinase 1/2 (ERK1/2) were investigated in colchicine-induced apoptosis on cortical neurons for the first time. Our results showed that 1 microM colchicine administration in primarily cultured cortical neurons led to typical neuronal apoptosis, and the apoptosis was attenuated by taxol, a microtubule stabilizer. Moreover, activation of p38 MAPK was found for the first time, as well as that of JNK MAPK, but not of ERK1/2 MAPK, after colchicine exposure. Apoptosis was inhibited by p38 MAPK inhibitors, SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole), SB239063 (trans-1-(4-hydroxycyclohexyl)-4-(fluorophenyl)-5-(2-methoxypyrimidin-4-yl) imidazole), and JNK MAPK pathway inhibitors, CEP11004 (9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid, 2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-5,16-bis[[(1-methylethyl)thio]methyl]-1-oxo-, methyl ester, (9S,10R,12R)-), SP600125 (anthra[1,9-cd]pyrazol-6(2H)-one). However, PD98059 (2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one) and U0126 (1,4-diamino-2,3-dicyano-1, 4-bis[2-aminophenylthio]butadiene), ERK1/2 MAPK inhibitors, did not work. Furthermore, better neuronal protective effects were achieved by using JNK and the p38 MAPK inhibitors together as compared to that by using either alone. The results suggested that p38 MAPK, JNK MAPK, but not ERK1/2 MAPK may play pivotal role in colchicine's neurotoxicity in primarily cultured cortical neurons, and the protective effects of the inhibition of p38 or JNK MPAK on cortical neurons were synergistically.

Glycogen synthase kinase-3beta induces neuronal cell death via direct phosphorylation of mixed lineage kinase 3

Mixed lineage kinase 3 (MLK3) is a mitogen-activated protein kinase kinase kinase member that activates the c-Jun N-terminal kinase (JNK) pathway. Aberrant activation of MLK3 has been implicated in neurodegenerative diseases. Similarly, glycogen synthase kinase (GSK)-3beta has also been shown to activate JNK and contribute to neuronal apoptosis. Here, we show a functional interaction between MLK3 and GSK-3beta during nerve growth factor (NGF) withdrawal-induced cell death in PC-12 cells. The protein kinase activities of GSK-3beta, MLK3, and JNK were increased upon NGF withdrawal, which paralleled increased cell death in NGF-deprived PC-12 cells. NGF withdrawal-induced cell death and MLK3 activation were blocked by a GSK-3beta-selective inhibitor, kenpaullone. However, the MLK family inhibitor, CEP-11004, although preventing PC-12 cell death, failed to inhibit GSK-3beta activation, indicating that induction of GSK-3beta lies upstream of MLK3. In GSK-3beta-deficient murine embryonic fibroblasts, ultraviolet light was unable to activate MLK3 kinase activity, a defect that was restored upon ectopic expression of GSK-3beta. The activation of MLK3 by GSK-3beta occurred via phosphorylation of MLK3 on two amino acid residues, Ser(789) and Ser(793), that are located within the C-terminal regulatory domain of MLK3. Furthermore, the cell death induced by GSK-3beta was mediated by MLK3 in a manner dependent on its phosphorylation of the specific residues within the C-terminal domain by GSK-3beta. Taken together, our data provide a direct link between GSK-3beta and MLK3 activation in a neuronal cell death pathway and identify MLK3 as a direct downstream target of GSK-3beta. Inhibition of GSK-3 is thus a potential therapeutic strategy for neurodegenerative diseases caused by trophic factor deprivation.

Expression and function of mixed lineage kinases in dendritic cells

Dendritic cells (DCs) sense the presence of conserved microbial structures in their local microenvironment via specific pattern recognition receptors (PRRs). This leads to a programme of changes, which include migration and activation, and enables them to induce adaptive T cell immunity. Mitogen-activated protein kinases (MAPKs) are implicated in this response, but the pathways leading from PRR ligation to MAPK activation, and hence DC activation, are not fully understood. Recent studies in the nervous system have suggested that the mixed lineage kinase (MLK) family of MAPK kinase kinase proteins may be involved as an intermediary step between PRRs and MAPKs. Therefore, in this study, we have used a well-established DC model to explore the role of MLKs in these cells. Messenger RNA for MLKs 2, 3, 4 and DLK and protein for MLKs 2, 3 and DLK are found in DC. DC activation in response to model PRR ligands, such as LPS or poly (I:C), is accompanied by phosphorylation of MLK3. In contrast, another known PRR ligand, zymosan, induces little MLK3 phosphorylation. Inhibition of MLK activity using a pharmacological inhibitor, CEP11004, blocks p38 and Jun N-terminal kinase (JNK) MAPK activation in response to LPS and poly (I:C), but not zymosan. The inhibition is associated with a block in DC activation as measured by cell-surface marker expression and cytokine secretion. Thus, MLKs are expressed in DC, and are implicated in DC activation, and the involvement of MLKs appears to be selective, depending on the nature of the DC stimulus.

Inhibition of mitogen-activated protein kinase and stimulation of Akt kinase signaling pathways: Two approaches with therapeutic potential in the treatment of neurodegenerative disease

The neurodegenerative diseases of adulthood, including Alzheimer's disease (AD) and Parkinson's disease (PD), pose an enormous and growing public health burden. Although effective symptomatic treatments exist for PD, and, to a lesser extent, for AD, there is no therapy for these disorders which will forestall their progression. With the rise of the concept of programmed cell death (PCD) came the realization that even in the absence of complete knowledge of proximate causes neuroprotection may nevertheless be possible by targeting the pathways of PCD. One set of signaling pathways that have been implicated in cell death are the mitogen-activated protein kinase (MAPK) pathways. The possibility of blocking these pathways and thereby providing neuroprotection has recently been put to the test in a clinical trial of a mixed lineage kinase inhibitor in the treatment of PD. Unfortunately, this trial failed to demonstrate a protective effect. Based on considerations related to the implementation of the trial, it would be premature to conclude that inhibition of MAPK signaling is a failed strategy. In spite of these negative results, the MAPK and related kinase pathways retain their importance as potential targets in PD. In relation to pathogenesis, the discovery of mutations in the mixed lineage kinase (MLK)-like kinase leucine-rich repeat kinase 2 (LRRK2) suggests a role for these kinases in regulating the viability of dopamine neurons. In relation to treatment, the survival signaling kinase Akt has been demonstrated in vivo to mediate striking neurotrophic and antiapoptotic effects. Thus, it is likely that therapeutic targets related to these kinase signaling pathways will emerge.

Mixed lineage kinases (MLKs): a role in dendritic cells, inflammation and immunity?

This review summarizes current knowledge about the mixed lineage kinases (MLKs) and explores their potential role in inflammation and immunity. MLKs were identified initially as signalling molecules in the nervous system. They were also shown to play a role in the cell cycle. Further studies documented three groups of MLKs, and showed that they may be activated via the c-Jun NH(2) terminal kinase (JNK) pathway, and by Rho GTPases. The biochemistry of the MLKs has been investigated in considerable detail. Homodimerization and heterodimerization can occur, and both autophosphorylation and autoinhibition are seen. The interaction between MLKs and JNK interacting protein (JIP) scaffolds, and the resultant effects on mitogen activated protein kinases, have been identified. Clearly, there is some redundancy within the MLK pathway(s), since mice which lack the MLK3 molecule are not abnormal. However, using a combination of biochemical analysis and pharmacological inhibitors, several recent studies in vitro have suggested that MLKs are not only expressed in cells of the immune system (as well as in the nervous system), but also may be implicated selectively in the signalling pathway that follows on toll-like receptor ligation in innate sentinel cells, such as the dendritic cell.

Jun N-terminal protein kinase enhances middle ear mucosal proliferation during bacterial otitis media

Mucosal hyperplasia is a characteristic component of otitis media. The present study investigated the participation of signaling via the Jun N-terminal protein kinase (JNK) mitogen-activated protein kinase in middle ear mucosal hyperplasia in animal models of bacterial otitis media. Otitis media was induced by the inoculation of nontypeable Haemophilus influenzae into the middle ear cavity. Western blotting revealed that phosphorylation of JNK isoforms in the middle ear mucosa preceded but paralleled mucosal hyperplasia in this in vivo rat model. Nuclear JNK phosphorylation was observed in many cells of both the mucosal epithelium and stroma by immunohistochemistry. In an in vitro model of primary rat middle ear mucosal explants, bacterially induced mucosal growth was blocked by the Rac/Cdc42 inhibitor Clostridium difficile toxin B, the mixed-lineage kinase inhibitor CEP11004, and the JNK inhibitor SP600125. Finally, the JNK inhibitor SP600125 significantly inhibited mucosal hyperplasia during in vivo bacterial otitis media in guinea pigs. Inhibition of JNK in vivo resulted in a diminished proliferative response, as shown by a local decrease in proliferating cell nuclear antigen protein expression by immunohistochemistry. We conclude that activation of JNK is a critical pathway for bacterially induced mucosal hyperplasia during otitis media, influencing tissue proliferation.

Endoplasmic reticulum stress-induced apoptosis requires bax for commitment and Apaf-1 for execution in primary neurons

Apoptosis triggered by endoplasmic reticulum (ER) stress is associated with various pathophysiological conditions including neurodegenerative diseases and ischemia. However, the mechanism by which ER stress induces neuronal apoptosis remains controversial. Here we identify the pathway of apoptosis carried out in sympathetic neurons triggered to die by ER stress-inducing agent tunicamycin. We find that ER stress induces a neuronal apoptotic pathway which upregulates BH3-only genes DP5 and Puma. Importantly, we show that ER stress commits neurons to die before cytochrome c release and this commitment requires Bax activation and c-jun N-terminal kinase signaling. Furthermore, ER stress engages the mitochondrial pathway of death as neurons release cytochrome c and Apaf-1 deficiency is sufficient to block apoptosis. Our findings identify a critical function of Bax in committing neurons to ER stress-induced apoptosis and clarify the importance of the apoptosome as the non-redundant caspase activation pathway to execute neuronal apoptosis in response to ER stress.

Synthesis and Mixed Lineage Kinase Activity of Pyrrolocarbazole and Isoindolone Analogs of (+)K-252a

Structural modification of the indolecarbazole natural product (+)K-252a identified structural requirements for MLK activity and a novel series of potent fused pyrrolocarbazole MLK1/3 inhibitors. The SAR revealed that the lactam regiochemistry, the shape of the heterocycle, and aryl rings B and F are important to MLK activity. Heteroatom and alkyl replacement of the N-12 and/or N-13 indole nitrogen atoms identified the nonplanar dihydronaphthyl[3,4-a]pyrrolo[3,4-c]carbazole-7-one (8) and corresponding 5,7-dione (7) as potent cell-permeable MLK1/3 family-selective leads with in vitro activity comparable to that of (+)K-252a and determined them to be 2- to 3-fold more potent than the aglycone natural product K-252c.

Kinase signaling pathways: potential therapeutic targets in Parkinson’s disease

There is substantial preclinical evidence that the mixed-lineage kinase–JNK–c-jun signaling pathway plays a role in programmed cell death in neurons, including dopamine neurons of the substantia nigra, which degenerate in Parkinson’s disease. Therefore, this pathway is a likely target for therapeutics. However, a recent large, double-blind clinical trial, the Parkinson Research Examination of CEP-1347 Trial (PRECEPT), of a mixed-lineage kinase inhibitor, CEP1347, has failed to demonstrate efficacy in preventing the progression of the disease, and has brought this approach to a crossroads. There are many considerations related to the implementation of the trial and our limited knowledge about the neural basis of the progression of the disease, which preclude a conclusion from this negative outcome that inhibition of this kinase pathway is a failed strategy. Furthermore, growing knowledge of the molecular basis of Parkinson’s disease, derived from investigations of genetic causes, including mutations in DJ-1, PTEN-induced putative kinase 1 (PINK-1) and leucine-rich repeat kinase 2 (LRRK2), suggest that kinase signaling pathways may be central to pathogenesis. We conclude that these pathways remain important future therapeutic targets.

JNK signaling in neomycin-induced vestibular hair cell death

Mechanosensory hair cells are susceptible to apoptotic death in response to exposure to ototoxic drugs, including aminoglycoside antibiotics. The c-Jun n-terminal kinase (JNK) is a stress-activated protein kinase that can promote apoptotic cell death in a variety of systems. Inhibition of the JNK signaling pathway can prevent aminoglycoside-induced death of cochlear and vestibular sensory hair cells. We used an in vitro preparation of utricles from adult mice to examine the role of JNK activation in aminoglycoside-induced hair cell death. CEP-11004 was used as an indirect inhibitor of JNK signaling. Immunohistochemistry showed that both JNK and its downstream target c-Jun are phosphorylated in hair cells of utricles exposed to neomycin. CEP-11004 inhibited neomycin-induced phosphorylation of both JNK and c-Jun. CEP-11004 inhibited hair cell death in utricles exposed to moderate doses of neomycin. However, the results were not uniform across the dose-response function; CEP-11004 did not inhibit hair cell death in utricles exposed to high-dose neomycin. The CEP-11004-induced protective effect was not due to inhibition of PKC or p38, since neither Chelerythrine nor SB203580 could mimic the protective effect of CEP-11004. In addition, inhibition of JNK inhibited the activation of caspase-9 in hair cells. These results indicate that JNK plays an important role in neomycin-induced vestibular hair cell death and caspase-9 activation.

Simplified staurosporine analogs as potent JAK3 inhibitors

Simplification of bottom ring and regioselective functionalization of the indolocarbazole unit of staurosporine (2) are described. The modification led to a new series of simplified staurosporine analogs, which exhibited significant inhibitory activity against Janus kinase 3 (JAK3). The structure-activity relationships (SAR) are discussed and a proposed binding model is also highlighted.

Indolocarbazole natural products: occurrence, biosynthesis, and biological activity

The indolocarbazole family of natural products, including the biosynthetically related bisindolylmaleimides, is reviewed (with 316 references cited). The isolation of indolocarbazoles from natural sources and the biosynthesis of this class of compounds are thoroughly reviewed, including recent developments in molecular genetics, enzymology and metabolic engineering. The biological activities and underlying modes of action displayed by natural and synthetic indolocarbazoles is also presented, with an emphasis on the development of analogs that have entered clinical trials for its future use against cancer or other diseases.

JNK pathway as therapeutic target to prevent degeneration in the central nervous system

JNKs (c-Jun N- terminal kinases) are important transducing enzymes involved in many faces of cellular regulation such as gene expression, cell proliferation and programmed cell death. The activation of JNK pathway is critical for naturally occurring neuronal death during development as well as for pathological death of adult brain following different insults. In particular, JNKs play an important role in excitotoxicity and all related phenomena. Initial research concentrated on defining the components and organization of JNK signalling cascades, but more recent studies have begun to see JNK as the appropriate target for prevent cell loss. We used a specific JNK inhibitor, the cell permeable peptide D-JNKI1, to block JNK action in neuronal death following excitotoxicity in vitro and cerebral ischemia in vivo. Here we review our recent findings and we discuss the possibility of using D-JNKI1 as a therapeutic agent to prevent cell loss in the central nervous system.

An inhibitor of c-Jun N-terminal kinases (CEP-11004) counteracts the anti-HIV-1 action of trichosanthin

Trichosanthin (TCS) is a type I ribosome-inactivating protein possessing multiple biological and pharmacological activities. One of its major actions is inhibition of human immunodeficiency virus (HIV) replication. The mechanism is still not clear. It is generally believed that this action is mediated via ribosome inactivation. Recently, we found that some TCS mutants with full ribosome inactivating activity were devoid of anti-HIV-1 effect. This suggested that there might be other mechanisms contributing to the anti-HIV-1 action. This study showed that a commonly used c-Jun N-terminal kinases inhibitor (CEP-11004) could counteract the antiviral action of TCS in C8166 cells. CEP-11004 alone had no effect on HIV-1 replication and TCS alone significantly inhibited this process. When CEP-11004 was used together with TCS, the antiviral action of TCS was much reduced. Two methods were used to assess viral replication. (1) By measuring the HIV-1 reverse transcriptase, TCS on the average reduced viral replication to 52+/-4%. With CEP-11004 pretreatment, TCS appeared to lose the HIV-1 inhibitory activity with viral replication stood at 101+/-7%. (2) By measuring HIV-1 p24, TCS reduced viral replication to 68+/-4%. With CEP-11004 pretreatment, TCS again seemed to lose its anti-HIV-1 activity with HIV-1 replication rose back to 101+/-4%. Both indexes indicated that CEP-11004 counteracted the antiviral action of TCS. Phosphorylation of JNK on the other hand was only slightly elevated by 1.5-fold by TCS and CEP-11004 inhibited this elevation. These results suggested that the anti-HIV-1 effect of TCS may be related to the MAPK signal process downstream from the point of CEP inhibition.

Regulation of axotomy-induced dopaminergic neuron death and c-Jun phosphorylation by targeted inhibition of cdc42 or mixed lineage kinase

Mechanical transection of the nigrostriatal dopamine pathway at the medial forebrain bundle (MFB) results in the delayed degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc). We have previously demonstrated that c-Jun activation is an obligate component of neuronal death in this model. Here we identified the small GTPase, cdc42, and mixed lineage kinases (MLKs) as upstream factors regulating neuronal loss and activation of c-Jun following MFB axotomy. Adenovirus-mediated expression of a dominant-negative form of cdc42 in nigral neurons blocked MFB axotomy-induced activation (phosphorylation) of MAP kinase kinase 4 (MKK4) and c-Jun, resulting in attenuation of SNpc neuronal death. Pharmacological inhibition of MLKs, MKK4-activating kinases, significantly reduced the phosphorylation of c-Jun and abrogated dopaminergic neuronal degeneration following MFB axotomy. Taken together, these findings suggest that death of nigral dopaminergic neurons following axotomy can be attenuated by targeting cell signaling events upstream of c-Jun N-terminal mitogen-activated protein kinase/c-Jun.

Role of extracellular-regulated kinase and c-jun NH2-terminal kinase in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurofilament phosphorylation

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes selective degeneration of dopaminergic neurons in which the c-Jun NH2-terminal kinase (JNK) signalling cascade has been implicated. We have employed a differentiated mouse neuroblastoma N2a cell model to investigate the involvement of JNK and extracellular-regulated kinase (ERK) in MPTP-mediated toxicity and their role in neurofilament heavy chain (NF-H) phosphorylation. Acute treatment with a cytotoxic MPTP concentration (5 mM) caused rapid and sustained JNK phosphorylation and ERK dephosphorylation, accompanied by cell death. In contrast, subcytotoxic concentrations of 10 microM MPTP resulted in lower, transient JNK activation in the presence of sustained ERK activity. This resulted in an aberrant increase in a phosphorylation-dependent NF-H epitope, perikaryal accumulation of NF-H, and loss of axon-like processes, prior to cell death. Inhibition of MEK kinase, using PD98059, showed that MEK 1/2 or the downstream kinase, ERK, is required for N2a cell differentiation, NF-H phosphorylation and survival. Indeed, MPTP-induced cell death was exacerbated by the presence of PD98059. However, in the presence of MPTP, reducing JNK activity by using an upstream specific mixed-lineage kinase inhibitor (CEP-11004) significantly attenuated aberrant NF-H phosphorylation and perikaryal NF-H accumulation and maintained axon-like processes, in addition to attenuating cell death. This study reports a switch in the predominant kinase involved in NF phosphorylation in a neuronal cell model and may have implications for the formation of inclusions. Our studies provide further evidence that modulation of the JNK pathway could have a role in alleviating neuronal cell death.

Inhibition of mixed-lineage kinase (MLK) activity during G2-phase disrupts microtubule formation and mitotic progression in HeLa cells

The mixed-lineage kinases (MLK) are serine/threonine protein kinases that regulate mitogen-activated protein (MAP) kinase signaling pathways in response to extracellular signals. Recent studies indicate that MLK activity may promote neuronal cell death through activation of the c-Jun NH2-terminal kinase (JNK) family of MAP kinases. Thus, inhibitors of MLK activity may be clinically useful for delaying the progression of neurodegenerative diseases, such as Parkinson's. In proliferating non-neuronal cells, MLK may have the opposite effect of promoting cell proliferation. In the current studies we examined the requirement for MLK proteins in regulating cell proliferation by examining MLK function during G2 and M-phase of the cell cycle. The MLK inhibitor CEP-11004 prevented HeLa cell proliferation by delaying mitotic progression. Closer examination revealed that HeLa cells treated with CEP-11004 during G2-phase entered mitosis similar to untreated G2-phase cells. However, CEP-11004 treated cells failed to properly exit mitosis and arrested in a pro-metaphase state. Partial reversal of the CEP-11004 induced mitotic arrest could be achieved by overexpression of exogenous MLK3. The effects of CEP-11004 treatment on mitotic events included the inhibition of histone H3 phosphorylation during prophase and prior to nuclear envelope breakdown and the formation of aberrant mitotic spindles. These data indicate that MLK3 might be a unique target to selectively inhibit transformed cell proliferation by disrupting mitotic spindle formation resulting in mitotic arrest.

Mutually exclusive subsets of BH3-only proteins are activated by the p53 and c-Jun N-terminal kinase/c-Jun signaling pathways during cortical neuron apoptosis induced by arsenite

The c-Jun N-terminal protein kinase (JNK)/c-Jun and p53 pathways form distinct death-signaling modules in neurons that culminate in Bax-dependent apoptosis. To investigate whether this signaling autonomy is due to recruitment of particular BH3-only proteins, we searched for a toxic signal that would activate both pathways in the same set of neurons. We show that arsenite activates both the JNK/c-Jun and p53 pathways in cortical neurons, which together account for >95% of apoptosis, as determined by using the mixed-lineage kinase (JNK/c-Jun) pathway inhibitor CEP11004 and p53-null mice. Despite the coexistence of both pathways in at least 30% of the population, Bim mRNA and protein expression was increased only by the JNK/c-Jun signaling pathway, whereas Noxa and Puma mRNA and Puma protein expression was entirely JNK/c-Jun independent. About 50% of Puma/Noxa expression was p53 dependent, with the remaining signal being independent of both pathways and possibly facilitated by arsenite-induced reduction in P-Akt. However, functionally, Puma was predominant in mediating Bax-dependent apoptosis, as evidenced by the fact that more than 90% of apoptosis was prevented in Puma-null neurons, although Bim was still upregulated, while Bim- and Noxa-null neurons died similarly to wild-type neurons. Thus, the p53 and JNK/c-Jun pathways can activate mutually exclusive subclasses of BH3-only proteins in the same set of neurons. However, other factors besides expression may determine which BH3-only proteins mediate apoptosis.

Mutant huntingtin alters MAPK signaling pathways in PC12 and striatal cells: ERK1/2 protects against mutant huntingtin-associated toxicity

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by an expanded polyglutamine (polyQ) tract within the huntingtin protein (Htt). Identifying the pathways that are altered in response to the mutant protein is crucial for understanding the cellular processes impacted by the disease as well as for the rational development of effective pharmacological interventions. Here, expression profiling of a cellular HD model identifies genes that implicate altered mitogen-activated protein kinase (MAPK) signaling. Targeted biochemical studies and pharmacological modulation of these MAPK pathways suggest that mutant Htt affects signaling at upstream points such that both ERK and JNK are activated. Modulation of the ERK pathway suggests that this pathway is associated with cell survival, whereas inhibition of JNK was found to effectively suppress pathogenesis. These studies suggest that pharmacological intervention in MAPK pathways, particularly at the level of ERK activation, may be an appropriate approach to HD therapy.

Treatment of Parkinson's disease : what's on the horizon?

Few neurological diseases have received as much attention and investment in research as Parkinson's disease. Although great strides have been made in the development of agents to treat this neurodegenerative disease, none yet address the underlying problem associated with it, the progressive loss of dopaminergic neurons. Current therapeutic strategies for Parkinson's disease focus primarily on reducing the severity of its symptoms using dopaminergic medications. Although providing substantial benefit, these agents are burdened by adverse effects and long-term complications. This review highlights new and emerging therapies for Parkinson's disease, categorised as symptomatic, neuroprotective and neurorestorative, although at times, this distinction is not easily made. Novel symptomatic treatments target nondopaminergic areas in the hope of avoiding the motor complications seen with dopaminergic therapies. Two emerging treatment approaches under investigation are adenosine A(2A) receptor antagonists (such as istradefylline [KW-6002]) and glutamate AMPA receptor antagonists (such as talampanel [LY-300164]). In 2003, the results from two studies using istradefylline in patients with Parkinson's disease were published, with both showing a positive benefit of the study drug when used as adjunctive therapy to levodopa. In non-human primate models of Parkinson's disease, talampanel has been found to have antiparkinsonian effects when administered as high-dose monotherapy and antidyskinetic effects on levodopa-induced dyskinesias. NS-2330, another drug currently undergoing clinical trials, is a triple monoamine reuptake inhibitor that has therapeutic potential in both Parkinson's and Alzheimer's disease. A phase II proof-of-concept study is currently underway in early Parkinson's disease. However, a recently published study in advanced Parkinson's disease showed no therapeutic benefit of NS-2330 in this patient population. Even more exciting are agents that have a neuroprotective or neurorestorative role. These therapies aim to prevent disease progression by targeting the mechanisms involved in the pathogenesis of Parkinson's disease. Several lines of investigation for neuroprotective therapies have been taken, including the antioxidant coenzyme Q10 (ubidecarenone) and anti-apoptotic agents such as CEP-1347. Studies in patients with Parkinson's disease with coenzyme Q10 have suggested that it slows down functional decline. The PRECEPT study is currently in progress to assess the neuroprotective role of CEP-1347 in the early phase of the disease. Gene therapy is another exciting arena and includes both potentially neuroprotective and neurorestorative agents. Novel methods include subthalamic glutamic acid decarboxylase gene therapy and the use of glial cell line-derived neurotrophic factor (GDNF). Eleven of 12 patients have been enrolled in the first FDA-approved phase I subthalamic glutamic acid decarboxylase gene therapy trial for Parkinson's disease, with currently no evidence of adverse events. GDNF delivered intracerebroventricularly was studied in a small population of patients with Parkinson's disease, but unfortunately did not reveal positive results. Other methods of administering GDNF include direct delivery via infusions into the basal ganglia and the use of viral vectors; thus far, these approaches have shown promising results. This is an exciting and rewarding time for research into Parkinson's disease. With so many therapies currently under investigation, the time is ripe for the beginning of a new phase of treatment strategies.

Regulation of gene expression by the amyloid precursor protein: inhibition of the JNK/c-Jun pathway

The amyloid precursor protein (APP) has been suggested to regulate gene expression. GeneChip analysis and in vitro kinase assays revealed potent APP-dependent repression of c-Jun, its target gene SPARC and reduced basal c-Jun N-terminal kinase (JNK) activity in PC12 cells overexpressing APP. UV-induced activation of the JNK signalling pathway and subsequent apoptosis were likewise reduced by APP and this effect could be mimicked by the indirect JNK inhibitor CEP-11004. Treatment with a gamma-secretase inhibitor did not affect APP-mediated downmodulation of the JNK signalling pathway, suggesting that the effects might be mediated via alpha-secretase processing of APP. In support of these data, overexpression of the Swedish mutant of APP did not inhibit SPARC expression, UV-induced JNK activation and cell death. Our data suggest an important physiological role of APP and alpha-secretase activity in the control of JNK/c-Jun signalling, target gene expression and cell death activation in response to cytotoxic stress.

CEP-11004, an inhibitor of the SAPK/JNK pathway, reduces TNF-alpha release from lipopolysaccharide-treated cells and mice

CEP-11004, a mixed lineage kinase (MLK) inhibitor, was examined for its effects on tumor necrosis factor-alpha (TNF-alpha) production in human THP-1 monocytes, mouse BV-2 microglia, and C57Bl/6 mice. CEP-11004 inhibited TNF-alpha secretion up to 90% in THP-1 cells incubated with 3 mug/ml lipopolysaccharide, with an IC50 of 137+/-14 nM. CEP-11004 also inhibited TNF-alpha production in lipopolysaccharide-stimulated microglial cells, but did not inhibit the initial increase in TNF-alpha mRNA expression as measured by real-time polymerase chain reaction (PCR). The mitogen-activated protein kinases (MAPKs) phospho-c-jun N-terminal kinase (JNK), phospho-p38, and phospho-MAPK kinase 4 (MKK4) levels were increased in THP-1 cells following lipopolysaccharide treatment, and were reduced by CEP-11004 treatment. For in vivo studies, CEP-11004 was injected 2 h prior to lipopolysaccharide (20 mg/kg) administration. CEP-11004 significantly inhibited TNF-alpha production at doses of 1-10 mg/kg as measured by enzyme-linked immunosorbent assay (ELISA). These results suggest that MLK blockade may be useful in inhibiting pro-inflammatory cytokine production in a wide range of diseases.

JIP1 regulates neuronal apoptosis in response to stress

We examined if the relative expression of JNK-interacting protein 1 (JIP1) and phosphorylated c-Jun N-terminal kinase (JNK) regulates cell signaling and contributes to selective neuronal vulnerability in response to environmental stress. In clonal neuroblastoma cultures, stresses such as hypoxia, ischemia, Abeta peptides, and UV irradiation rapidly reduced JIP1 expression. JIP1 mRNA expression was also down-regulated by UV stress and was accompanied by increased JNK and c-Jun activation and cell death. JIP1 protein reduction was partially reversed both by inhibitors predominantly of caspase 3 and of the JNK pathway and resulted in significantly increased cell survival. Conversely, overexpression of JIP1 decreased both nuclear translocation of activated-JNK, and c-Jun phosphorylation induced by either UV irradiation, or the JNK upstream activators, MKK7 or MEKK1. Cell death was reduced about 50% compared to GFP-transfected controls. JIP1 overexpression did not facilitate either JNK expression or activation. In the normal, non-stressed human hippocampus and rat hippocampal organotypic cultures, JIP1 and JNK3 were inversely expressed with more JIP1 in CA2 and CA3 and less in CA1 neurons. In the human hippocampus, transient hypoxia/ischemia selectively spares neurons in CA2 and CA3 and induces death of neurons in the hippocampal CA1 subregion. In the cultures, ischemia reduced JIP1 expression and activated JNK, c-Jun, and caspase 3. Inhibitors of the JNK pathway, JNK activation directly and of caspase 3 activation each partially reversed these effects. Thus, under certain stress conditions, down-regulation of JIP1 expression makes neurons more susceptible to apoptosis, suggesting JIP may serve as an anti-apoptosis factor.

The synthesis of bioactive indolocarbazoles related to K-252a

A range of functionalised indolocarbazoles, related to the natural product K-252a, have been prepared, starting from a readily available bridged cyclopentene. Sequences of transformations, involving initial hydroboration-oxidation to give a ketone, or by dihydroxylation and cyclic sulfate formation, enable the preparation of diverse indolocarbazole products. Issues of imide nitrogen protection for the indolocarbazole, and opportunities for asymmetric desymmetrisation of key intermediates were also explored. A novel chiral lithium amide base mediated transformation of a cyclic sulfate intermediate gave the anticipated ketone product in up to 87% ee. A number of compounds, in the form of unprotected imide substituted indolocarbazoles, were screened for biological activity and were found to be potent inhibitors of a number of kinase enzymes.

JNK activation limits dendritic cell maturation in response to reactive oxygen species by the induction of apoptosis

Dendritic cells (DC) sense infection in their local microenvironment and respond appropriately in order to induce T cell immunity. This response is mediated in part via the mitogen-activated protein kinase (MAPK) pathways. Hydrogen peroxide is present frequently in the inflammatory DC milieu and is known to activate MAPK. Therefore this study examines the role of hydrogen peroxide, both alone and in combination with lipopolysaccharide (LPS), in the regulation of activation of two key MAPK, p38 and JNK, regulation of phenotype, and regulation of apoptosis in human monocyte-derived DC. At low concentrations, hydrogen peroxide activates p38, but does not alter DC phenotype. At higher concentrations, hydrogen peroxide activates both p38 and JNK. Activation of JNK, which is associated with inhibition of tyrosine phosphatases in DC, is linked to the induction of DC apoptosis. An upstream JNK inhibitor (CEP11004) and a competitive JNK inhibitor (SP600125) both partially protected the DC from the proapoptotic effects of hydrogen peroxide. Unexpectedly, hydrogen peroxide and LPS synergize in inducing JNK activation and DC apoptosis. JNK-mediated apoptosis may limit damaging immune responses against neoepitopes generated by modification of self-antigens by reactive oxygen species present at sites of inflammation.

Sensitization of Glioma Cells to Fas-Dependent Apoptosis by Chemotherapy-Induced Oxidative Stress

A prominent feature of glioblastoma is its resistance to death from Fas pathway activation. In this study, we explored the modulation of Fas-induced glioblastoma death with chemotherapeutic agents. Camptothecin significantly increased the glioblastoma cell death response to Fas receptor activation regardless of p53 status. Sublethal concentrations of camptothecin reduced the IC50 of agonistic anti-Fas antibody (CH-11) 10-fold, from 500 to 50 ng/mL, in human U87 glioblastoma cells (p53 wild-type). Cell viability in response to camptothecin, CH-11 alone, and the combination of camptothecin + CH-11 was found to be 84%, 85%, and 47% (P < 0.001), respectively. A similar pattern of relative cytotoxicity was found in U373 cells (p53 mutant). We further examined the pathways and mechanisms involved in this apparent synergistic cytotoxic response. Cell death was found to be predominantly apoptotic involving both extrinsic and intrinsic pathways as evidenced by annexin V staining, cleavage of caspases (3, 8, and 9), increased caspase activities, Smac release, and cytoprotection by caspase inhibitors. Expression of Fas-associated death domain, and not Fas, Fas ligand, or caspase proteins, increased following cell treatment with camptothecin + CH-11. Camptothecin treatment enhanced c-jun-NH2-kinase activation in response to CH-11, but inhibition of c-jun-NH2-kinase did not prevent cell death induced by the combination treatment. Reactive oxygen species, especially H2O2, were elevated following camptothecin treatment; and H2O2 enhanced cell death induced by CH-11. The antioxidants glutathione and N-acetyl-cysteine prevented cell death induced by camptothecin + CH-11. These findings show that camptothecin synergizes with Fas activation to induce glioblastoma apoptosis via a mechanism involving reactive oxygen species and oxidative stress pathways.

Synthesis, Modeling, and In Vitro Activity of (3‘S)-epi-K-252a Analogues. Elucidating the Stereochemical Requirements of the 3‘-Sugar Alcohol on trkA Tyrosine Kinase Activity

Utilizing our recently published semisynthetic approach to the (3'S)-K-252a diastereomer, we report the first synthesis of the (3'R)-10 diastereomer and a set of related epimers, with the goal of defining the stereochemical role of the 3'-sugar hydroxyl group on trkA tyrosine kinase activity and selectivity. (3'R)-10 displayed potent trkA inhibitory activity with an IC50 value of 4 nM. The corresponding deshydroxy epimer (3'S)-14 was 7-fold more potent than its 3'R counterpart (natural stereochemistry) with a trkA IC50 value of 3 nM and demonstrated >280-fold selectivity over PKC (IC50 = 850 nM). In cells, (3'S)-14 displayed potent inhibition of trkA autophosphorylation with an IC50 < 10 nM. Molecular modeling studies revealed that the 3'-OH, due to the inverted geometry, forms significant H-bonding interactions with Glu27 and Arg195, an interaction that is not attainable with the natural isomers.

Complete Inhibition of Anisomycin and UV Radiation but Not Cytokine Induced JNK and p38 Activation by an Aryl-substituted Dihydropyrrolopyrazole Quinoline and Mixed Lineage Kinase 7 Small Interfering RNA

Mixed lineage kinase 7 (MLK7) is a mitogen-activated protein kinase kinase kinase (MAPKKK) that activates the pro-apoptotic signaling pathways p38 and JNK. A library of potential kinase inhibitors was screened, and a series of dihydropyrrolopyrazole quinolines was identified as highly potent inhibitors of MLK7 in vitro catalytic activity. Of this series, an aryl-substituted dihydropyrrolopyrazole quinoline (DHP-2) demonstrated an IC50 of 70 nM for inhibition of pJNK formation in COS-7 cell MLK7/JNK co-transfection assays. In stimulated cells, DHP-2 at 200 nM or MLK7 small interfering RNA completely blocked anisomycin and UV induced but had no effect on interleukin-1beta or tumor necrosis factor-alpha-induced p38 and JNK activation. Additionally, the compound blocked anisomycin and UV-induced apoptosis in COS-7 cells. Heart tissue homogenates from MLK7 transgenic mice treated with DHP-2 at 30 mg/kg had reduced JNK and p38 activation with no apparent effect on ERK activation, demonstrating that this compound can be used to block MLK7-driven MAPK pathway activation in vivo. Taken together, these data demonstrate that MLK7 is the MAPKKK required for modulation of the stress-activated MAPKs downstream of anisomycin and UV stimulation and that DHP-2 can be used to block MLK7 pathway activation in cells as well as in vivo.