Complete nucleotide sequence, expression, and chromosomal localisation of human mixed-lineage kinase 2

The emerging role of mixed lineage kinase 3 (MLK3) and its potential as a target for neurodegenerative diseases therapies

Selective and brain-permeable protein kinase inhibitors are in preclinical development for treating neurodegenerative diseases. Among them, MLK3 inhibitors, with a potent neuroprotective biological action have emerged as valuable agents for the treatment of pathologies such as Alzheimer's, Parkinson's disease and amyotrophic lateral sclerosis. In fact, one MLK3 inhibitor, CEP-1347, reached clinical trials for Parkinson's disease. Additionally, another compound called prostetin/12k, a potent and rather selective MLK3 inhibitor has started clinical development for ALS based on its motor neuron protection in both in vitro and in vivo models. In this review, we will focus on the role of MLK3 in neuron-related cell death processes, neurodegenerative diseases, and the potential advantages of targeting this kinase through pharmacological modulation for neuroprotective treatment.

The regulatory function of mixed lineage kinase 3 in tumor and host immunity

Protein kinases are the second most sought-after G-protein coupled receptors as drug targets because of their overexpression, mutations, and dysregulated catalytic activities in various pathological conditions. Till 2019, 48 protein kinase inhibitors have received FDA approval for the treatment of multiple illnesses, of which the majority of them are indicated for different malignancies. One of the attractive sub-group of protein kinases that has attracted attention for drug development is the family members of MAPKs that are recognized to play significant roles in different cancers. Several inhibitors have been developed against various MAPK members; however, none of them as monotherapy has shown sustainable efficacy. One of the MAPK members, called Mixed Lineage Kinase 3 (MLK3), has attracted considerable attention due to its role in inflammation and neurodegenerative diseases; however, its role in cancer is an emerging area that needs more investigation. Recent advances have shown that MLK3 plays a role in cancer cell survival, migration, drug resistance, cell death, and tumor immunity. This review describes how MLK3 regulates different MAPK pathways, cancer cell growth and survival, apoptosis, and host's immunity. We also discuss how MLK3 inhibitors can potentially be used along with immunotherapy for different malignancies.

The ERK cascade: a prototype of MAPK signaling

Sequential activation of protein kinases within the mitogen-activated protein kinase (MAPK) cascades is a common mechanism of signal transduction in many cellular processes. Four such cascades have been elucidated thus far, and named according to their MAPK tier component as the ERK1/2, JNK, p38MAPK, and ERK5 cascades. These cascades cooperate in transmitting various extracellular signals, and thus control cellular processes such as proliferation, differentiation, development, stress response, and apoptosis. Here we describe the classic ERK1/2 cascade, and concentrate mainly on the properties of MEK1/2 and ERK1/2, including their mode of regulation and their role in various cellular processes and in oncogenesis. This cascade may serve as a prototype of the other MAPK cascades, and the study of this cascade is likely to contribute to the understanding of mitogenic and other processes in many cell lines and tissues.

N-Acetylcysteine inhibit the translocation of mixed lineage kinase-3 from cytosol to plasma membrane during transient brain ischemia in rat hippocampus

Mixed lineage kinase-3 (MLK3) is a recently described member of the MLK subfamily of Ser/Thr protein kinases that interacts with mitogen-activated protein kinase (MAPK) pathways. In this study, we investigated the translocation of MLK3 during transient cerebral ischemia in rat hippocampus. Transient brain ischemia was induced by the four-vessel occlusion in Sprague-Dawley rats. Our data show that MLK3 can translocate from cytosolic fraction to the membrane fraction during ischemia and the increased MLK3 in the membrane fraction bind to postsynaptic density protein 95 (PSD-95). The antioxidant N-acetylcysteine (NAC) could inhibit the translocation of MLK3 from cytosolic fraction to the membrane fraction and decrease the interactions of MLK3 and PSD-95 in the membrane fraction. Consequently, these results indicate that reactive oxygen species (ROS) was closely associated with MLK3 translocation induced by transient global ischemia in rat hippocampus.

Diversity in domain architectures of Ser/Thr kinases and their homologues in prokaryotes

Background

Ser/Thr/Tyr kinases (STYKs) commonly found in eukaryotes have been recently reported in many bacterial species. Recent studies elucidating their cellular functions have established their roles in bacterial growth and development. However functions of a large number of bacterial STYKs still remain elusive. The organisation of domains in a large dataset of bacterial STYKs has been investigated here in order to recognise variety in domain combinations which determine functions of bacterial STYKs.

Results

Using sensitive sequence and profile search methods, domain organisation of over 600 STYKs from 125 prokaryotic genomes have been examined. Kinase catalytic domains of STYKs tethered to a wide range of enzymatic domains such as phosphatases, HSP70, peptidyl prolyl isomerases, pectin esterases and glycoproteases have been identified. Such distinct preferences for domain combinations are not known to be present in either the Histidine kinase or the eukaryotic STYK families. Domain organisation of STYKs specific to certain groups of bacteria has also been noted in the current anlaysis. For example, Hydrophobin like domains in Mycobacterial STYK and penicillin binding domains in few STYKs of Gram-positive organisms and FHA domains in cyanobacterial STYKs. Homologues of characterised substrates of prokaryotic STYKs have also been identified.

Conclusion

The domains and domain architectures of most of the bacterial STYKs identified are very different from the known domain organisation in STYKs of eukaryotes. This observation highlights distinct biological roles of bacterial STYKs compared to eukaryotic STYKs. Bacterial STYKs reveal high diversity in domain organisation. Some of the modular organisations conserved across diverse bacterial species suggests their central role in bacterial physiology. Unique domain architectures of few other groups of STYKs reveal recruitment of functions specific to the species.

Phosphoregulation of mixed-lineage kinase 1 activity by multiple phosphorylation in the activation loop

Mixed-lineage kinase 1 (MLK1) is a mitogen-activated protein kinase kinase kinase capable of activating the c-Jun NH(2)-terminal kinase (JNK) pathway. Full-length MLK1 has 1104 amino acids and a domain structure identical to MLK2 and MLK3. Immunoblot and mass spectrometry show that MLK1 is threonine (and possibly serine) phosphorylated in or near the activation loop. A kinase-dead mutant is not, consistent with autophosphorylation. Mutation to alanine of any of the four serine or threonine residues in the activation loop reduces both the activity of the recombinant kinase domain and JNK pathway activation driven by full-length MLK1 expressed in mammalian cells. Furthermore, the gel mobility of the mutant MLK1s is closer to that of the kinase-dead than wild type, consistent with reduced phosphorylation. Thr312 is the key residue: MLK1[T312A] retains only basal activity (about 1-2% of wild type), and its gel mobility is indistinguishable from kinase-dead. Thr312 does not suffice, however; phosphorylation of multiple sites is necessary for full activation of MLK1. An activation mechanism consistent with these data involves phosphorylation of multiple sites in the activation loop, with phosphorylation of Thr312 required for full phosphorylation. This mechanism is broadly similar to that previously reported for MLK3 [Leung, I. W., and Lassam, N. (2001) J. Biol. Chem. 276, 1961-1967], but the key residue differs.

Mixed lineage kinase 3 (MLK3)-activated p38 MAP kinase mediates transforming growth factor-beta-induced apoptosis in hepatoma cells

Although transforming growth factor beta1 (TGF-beta1) acts via the Smad signaling pathway to initiate de novo gene transcription, the TGF-beta1-induced MAPK kinase activation that is involved in the regulation of apoptosis is less well understood. Even though the p38 MAP kinase and c-Jun NH(2)-terminal kinases (JNKs) are involved in TGF-beta1-induced cell death in hepatoma cells, the upstream mediators of these kinases remain to be defined. We show here that the members of the mixed lineage kinase (MLK) family (including MLK1, MLK2, MLK3, and dual leucine zipper-bearing kinase (DLK)) are expressed in FaO rat hepatoma cells and are likely to act between p38 and TGF-beta receptor kinase in death signaling. TGF-beta1 treatment leads to an increase in MLK3 activity. Overexpression of MLK3 enhances TGF-beta1-induced apoptotic death in FaO cells and Hep3B human hepatoma cells, whereas expression of the dominant-negative forms of MLK3 suppresses cell death induced by TGF-beta1. The dominant-negative forms of MLK1 and -2 also suppress TGF-beta1-induced cell death. In MLK3-overexpressing cells, ERK, JNKs, and p38 MAP kinases were further activated in response to TGF-beta1 compared with the control cells. In contrast, overexpression of the dominant-negative MLK3 resulted in suppression of TGF-beta1-induced MAP kinase activation and TGF-beta1-induced caspase-3 activation. We also show that only the inhibition of the p38 pathway suppressed TGF-beta1-induced apoptosis. These observations support a role for MLKs in the TGF-beta1-induced cell death mechanism.

MIXED-LINEAGEKINASES: A Target for the Prevention of Neurodegeneration

The activation of the c-Jun N-terminal kinase (JNK) pathway is critical for naturally occurring neuronal cell death during development and may be important for the pathological neuronal cell death of neurodegenerative diseases. The small molecule inhibitor of the mixed-lineage kinase (MLK) family of kinases, CEP-1347, inhibits the activation of the JNK pathway and, consequently, the cell death in many cell culture and animal models of neuronal death. CEP-1347 has the ability not only to inhibit cell death but also to maintain the trophic status of neurons in culture. The possible importance of the JNK pathway in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases provides a rationale for the use of CEP-1347 for the treatment of these diseases. CEP-1347 has the potential of not only retarding disease progression but also reversing the severity of symptoms by improving the function of surviving neurons.

Antioxidant N-acetylcysteine and AMPA/KA receptor antagonist DNQX inhibited mixed lineage kinase-3 activation following cerebral ischemia in rat hippocampus

We measured the MLK3 expression, activity and backphosphorylation following cerebral ischemia. Our data showed that MLK3 protein levels were unalterable during ischemia and reperfusion. However, during ischemia MLK3 activity gradually increased and reached its peak at 30 min of ischemia. While its backphosphorylation reduced from 5 min of ischemia to 30 min of ischemia. In addition, we also detected MLK3 alteration at various time points of reperfusion after 15 min of ischemia, which showed that MLK3 activity increased twice, whereas MLK3 backphosphorylation was similarly consistent with its activity during reperfusion. To further analyze the reason of MLK3 activation, antioxidant N-acetylcysteine (NAC) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate (AMPA)/kainate (KA) receptor antagonist 6,7-dinitroquinoxaline-2,3(1H, 4H)-dione (DNQX) were given to the rats 20 min prior to ischemia. The results illustrated that NAC preferably inhibited the MLK3 activation during the ischemia and the early reperfusion, whereas DNQX effectively attenuated the MLK3 activation of the late reperfusion. We think that MLK3 activation is certainly associated with reactive oxygen species (ROS) and AMPA/KA receptor in response to ischemic insult.

Systematic trans-genomic comparison of protein kinases between Arabidopsis and Saccharomyces cerevisiae

The genome of the budding yeast (Saccharomyces cerevisiae) provides an important paradigm for transgenomic comparisons with other eukaryotic species. Here, we report a systematic comparison of the protein kinases of yeast (119 kinases) and a reference plant Arabidopsis (1,019 kinases). Using a whole-protein-based, hierarchical clustering approach, the complete set of protein kinases from both species were clustered. We validated our clustering by three observations: (a) clustering pattern of functional orthologs proven in genetic complementation experiments, (b) consistency with reported classifications of yeast kinases, and (c) consistency with the biochemical properties of those Arabidopsis kinases already experimentally characterized. The clustering pattern identified no overlap between yeast kinases and the receptor-like kinases (RLKs) of Arabidopsis. Ten more kinase families were found to be specific for one of the two species. Among them, the calcium-dependent protein kinase and phosphoenolpyruvate carboxylase kinase families are specific for plants, whereas the Ca(2+)/calmodulin-dependent protein kinase and provirus insertion in mouse-like kinase families were found only in yeast and animals. Three yeast kinase families, nitrogen permease reactivator/halotolerance-5), polyamine transport kinase, and negative regulator of sexual conjugation and meiosis, are absent in both plants and animals. The majority of yeast kinase families (21 of 26) display Arabidopsis counterparts, and all are mapped into Arabidopsis families of intracellular kinases that are not related to RLKs. Representatives from 11 of the common families (54 kinases from Arabidopsis and 17 from yeast) share an extremely high degree of similarity (blast E value < 10(-80)), suggesting the likelihood of orthologous functions. Selective expansion of yeast kinase families was observed in Arabidopsis. This is most evident for yeast genes CBK1, HRR25, and SNF1 and the kinase family S6K. Reduction of kinase families was also observed, as in the case of the NEK-like family. The distinguishing features between the two sets of kinases are the selective expansion of yeast families and the generation of a limited number of new kinase families for new functionality in Arabidopsis, most notably, the Arabidopsis RLKs that constitute important components of plant intercellular communication apparatus.

Cross-talk between JNK/SAPK and ERK/MAPK pathways: sustained activation of JNK blocks ERK activation by mitogenic factors

Mixed lineage kinases (MLKs) are a family of serine/threonine kinases that function in the SAPK signaling cascade. MLKs activate JNK/SAPK in vivo by directly phosphorylating and activating the JNK kinase SEK-1 (MKK4 and -7). Importantly, the MLK member MLK3/SPRK has been shown recently to be a direct target of ceramide and tumor necrosis factor-alpha (TNF-alpha) and to mediate the TNF-alpha and ceramide-induced JNK activation in Jurkat cells. Here we report that MLK3 can phosphorylate and activate MEK-1 directly in vitro and also can induce MEK phosphorylation on its activation sites in vivo in COS-7 cells. Surprisingly, this induction of MEK phosphorylation does not result in ERK activation in vivo. Rather, in cells expressing active MLK3, ERK becomes resistant to activation by growth factors and mitogens. This restriction in ERK activation requires MLK3 kinase activity, is independent of Raf activation, and is reversed by JNK pathway inhibition either at the level of SEK-1, JNK, or Jun. These results demonstrate that sustained JNK activation uncouples ERK activation from MEK in a manner requiring Jun-mediated gene transcription. This in turn points to the existence of a negative cross-talk relationship between the stress-activated JNK pathway and the mitogen-activated ERK pathway. Thus, our findings imply that some of the biological functions of JNK activators, such as TNF-alpha and ceramide, may be attributed to their ability to block cell responses to growth and survival factors acting through the ERK/MAPK pathway.

Map kinase signaling pathways and hematologic malignancies

Mitogen-activated protein (Map) kinases are widely expressed serine-threonine kinases that mediate important regulatory signals in the cell. Three major groups of Map kinases exist: the p38 Map kinase family, the extracellular signal-regulated kinase (Erk) family, and the c-Jun NH2-terminal kinase (JNK) family. The members of the different Map kinase groups participate in the generation of various cellular responses, including gene transcription, induction of cell death or maintenance of cell survival, malignant transformation, and regulation of cell-cycle progression. Depending on the specific family isoform involved and the cellular context, Map kinase pathways can mediate signals that either promote or suppress the growth of malignant hematopoietic cells. Over the last few years, extensive work by several groups has established that Map kinase pathways play critical roles in the pathogenesis of various hematologic malignancies, providing new molecular targets for future therapeutic approaches. In this review, the involvement of various Map kinase pathways in the pathophysiology of hematologic malignances is summarized and the clinical implications of the recent advances in the field are discussed.

Drosophila mixed lineage kinase/slipper, a missing biochemical link in Drosophila JNK signaling

Mixed lineage kinases (MLKs) belong to the family of mitogen activated protein kinase kinase kinase (MAPKKK) and cause neuronal cell death mediated through c-Jun, N-terminal kinase (JNK) pathway. Recently, genetic studies in Drosophila revealed the presence of an MLK termed slipper (slpr). However, its biochemical features like physiological substrate, role in different MAPK pathways and developmental and tissue-specific expression pattern were not reported. Here, we report cDNA cloning, expression analysis and biochemical characterization of a Drosophila mixed lineage kinase (dMLK) that is also known as slipper. The protein structure analysis of dMLK/slipper revealed, in addition to the conserved domains, a stretch of glutamine in the amino terminus and an asparagine-threonine stretch at the carboxy-terminus. In situ hybridization and reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed that dMLK is expressed in early embryonic stages, adult brain and thorax. Ectopic expression of dMLK either in Drosophila S2 or in mammalian HEK293 cells leads to activation of JNK, p38 and extracellular signal regulated kinase (ERK) pathways. Further, dMLK directly phosphorylates Hep, dMKK4 and also their mammalian counterparts, MKK7 and SEK1, in an in vitro kinase assay. Taken together, our results provide for the first time a comprehensive expression profile and new biochemical insight of dMLK/slipper.

Neuronal p38 MAPK signalling: an emerging regulator of cell fate and function in the nervous system

p38 mitogen-activated protein kinases (MAPKs), together with extracellular signal-regulated kinases (ERKs) and c-Jun N-terminal kinases (JNKs), constitute the MAPK family. Multiple intracellular signalling pathways that converge on MAPKs exist in all eukaryotic cells and play pivotal roles in a wide variety of cellular functions. p38 MAPKs and JNKs, also termed stress-activated protein kinases (SAPKs), are preferentially activated by various cytotoxic stresses and cytokines and appear to be potent regulators of stress-induced apoptosis. Whereas JNKs have been shown to play pivotal roles in the regulation of neuronal apoptosis, the role of p38 MAPKs in the nervous system is poorly understood. However, accumulating evidence from mammalian cell culture systems and the strong genetic tool C. elegans suggests that neuronal p38 signalling has diverse functions beyond the control of cell death and survival. This review focuses on possible roles for the p38 pathway in the nervous system, with principal emphasis placed on the roles in neuronal cell fate decision and function.

Mixed lineage kinase 2 interacts with clathrin and influences clathrin-coated vesicle trafficking

Mixed lineage kinase 2 (MLK2) is a protein kinase that signals in the stress-activated Jun N-terminal kinase signal transduction pathway. We used immunoprecipitation and mass spectrometric analysis to identify MLK2-binding proteins in cell lines with inducible expression of green fluorescent protein-tagged MLK2. Here we report the identification of clathrin as a binding partner for MLK2 in both cultured cells and mammalian brain. We demonstrate that clathrin binding requires a motif (LLDMD) located near the MLK2 C terminus, which is similar to "clathrin box" motifs important for binding of clathrin coat assembly and accessory proteins to the clathrin heavy chain. A C-terminal fragment of MLK2 containing this motif binds strongly to clathrin, and mutation of the LLDMD sequence to LAAAD completely abrogates clathrin binding. We isolated clathrin-coated vesicles from green fluorescent protein-MLK2-expressing cells and from mouse brain lysates and found that MLK2 is enriched along with clathrin in these vesicles. In addition, we demonstrated that endogenous MLK2 co-immunoprecipitates with clathrin heavy chain from the vesicle-enriched fraction of mouse brain lysate. Furthermore, overexpression of MLK2 in cultured cells inhibits accumulation of labeled transferrin in recycling endosomes during receptor-mediated endocytosis. These findings suggest a role for MLK2 and the stress-signaling pathway at sites of clathrin activity in vesicle formation or trafficking.

Mixed-lineage kinase control of JNK and p38 MAPK pathways

Mixed-lineage kinases (MLKs) are serine/threonine protein kinases that regulate signalling by the c-Jun amino-terminal kinase (JNK) and p38 mitogen-activated-protein kinase (MAPK) pathways. MLKs are represented in the genomes of both Caenorhabditis elegans and Drosophila melanogaster. The Drosophila MLK Slipper regulates JNK to control dorsal closure during embryonic morphogenesis. In mammalian cells, MLKs are implicated in the control of apoptosis and are potential drug targets for many neurodegenerative diseases.

MRK, a mixed lineage kinase-related molecule that plays a role in gamma-radiation-induced cell cycle arrest

Mitogen-activated protein (MAP) kinase pathways are three-kinase modules that mediate diverse cellular processes and have been highly conserved among eukaryotes. By using a functional complementation screen in yeast, we have identified a human MAP kinase kinase kinase (MAPKKK) that shares homology with members of the mixed lineage kinase (MLK) family and therefore was called MRK (MLK-related kinase). We report the structure of the MRK gene, from which are generated two splice forms of MRK, MRK-alpha and MRK-beta, encoding for proteins of 800 and 456 amino acids, respectively. By using a combination of solid phase protein kinase assays, transient transfections in cells, and analysis of endogenous proteins in stably transfected Madin-Darby canine kidney cells, we found that MRK-beta preferentially activates ERK6/p38gamma via MKK3/MKK6 and JNK through MKK4/MKK7. We also show that expression of wild type MRK increases the cell population in the G(2)/M phase of the cell cycle, whereas dominant negative MRK attenuates the G(2) arrest caused by gamma-radiation. In addition, exposure of cells to gamma-radiation induces MRK activity. These data suggest that MRK may mediate gamma-radiation signaling leading to cell cycle arrest and that MRK activity is necessary for the cell cycle checkpoint regulation in cells.

Activation of the JNK pathway during dorsal closure in Drosophila requires the mixed lineage kinase, slipper

The Jun kinase (JNK) pathway has been characterized for its role in stimulating AP-1 activity and for modulating the balance between cell growth and death during development, inflammation, and cancer. Six families of mammalian kinases acting at the level of JNKKK have emerged as upstream regulators of JNK activity (MLK, LZK, TAK, ASK, MEKK, and TPL); however, the specificity underlying which kinase is utilized for transducing a distinct signal is poorly understood. In Drosophila, JNK signaling plays a central role in dorsal closure, controlling cell fate and cell sheet morphogenesis during embryogenesis. Notably, in the fly genome, there are single homologs of each of the mammalian JNKKK families. Here, we identify mutations in one of those, a mixed lineage kinase, named slipper (slpr), and show that it is required for JNK activation during dorsal closure. Furthermore, our results show that other putative JNKKKs cannot compensate for the loss of slpr function and, thus, may regulate other JNK or MAPK-dependent processes.

Autoinhibition of mixed lineage kinase 3 through its Src homology 3 domain

Mixed lineage kinase 3 (MLK3) is a serine/threonine protein kinase that functions as a mitogen-activated protein kinase kinase kinase to activate the c-Jun NH(2)-terminal kinase pathway. MLK3 has also been implicated as an I kappa B kinase kinase in the activation of NF-kappa B. Amino-terminal to its catalytic domain, MLK3 contains a Src homology 3 (SH3) domain. SH3 domains harbor three highly conserved aromatic amino acids that are important for ligand binding. In this study, we mutated one of these corresponding residues within MLK3 to deliberately disrupt the function of its SH3 domain. This SH3-defective mutant of MLK3 exhibited increased catalytic activity compared with wild type MLK3 suggesting that the SH3 domain negatively regulates MLK3 activity. We report herein that the SH3 domain of MLK3 interacts with full-length MLK3, and we have mapped the site of interaction to a region between the zipper and the Cdc42/Rac interactive binding motif. Interestingly, the SH3-binding region contains not a proline-rich sequence but, rather, a single proline residue. Mutation of this sole proline abrogates SH3 binding and increases MLK3 catalytic activity. Taken together, these data demonstrate that MLK3 is autoinhibited through its SH3 domain. The critical proline residue in the SH3-binding site of MLK3 is conserved in the closely related family members, MLK1 and MLK2, suggesting a common autoinhibitory mechanism among these kinases. Our study has revealed the first example of SH3 domain-mediated autoinhibition of a serine/threonine kinase and provides insight into the regulation of the mixed lineage family of protein kinases.

Tissue distribution and functional expression of a cDNA encoding a novel mixed lineage kinase

Hypertrophy is an adaptive response of the heart to myocardial injury or hemodynamic overload that may progress and contribute to cardiac decompensation and eventually to heart failure. The signaling pathways controlling this response in the cardiac myocyte are poorly understood. A data mining effort of a human failed heart cDNA library was undertaken in an effort to identify novel signaling molecules involved in cardiac hypertrophy. This effort identified a novel kinase (MLK7) homologous to the mixed lineage kinase family of proteins. The mixed lineage kinases are mitogen-activated protein kinase kinase kinases (MAPKKKs) which activate stress activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) and p38 kinase pathways. They contain a catalytic domain with homology to both serine/threonine and tyrosine-specific kinases and a dual leucine zipper. MLK7 is identical to leucine zipper and sterile-alpha motif protein kinase (ZAK) through the leucine zipper domain but has a completely divergent COOH-terminus and shares approximately 40% homology with the other MLKs overall. Expression of MLK7 mRNA is most abundant in skeletal muscle and heart, with expression restricted to the cardiac myocyte. The recombinant histidine tagged MLK7 expressed and purified from insect cells exhibited serine/threonine kinase activity in vitro with myelin basic protein as substrate. When expressed in cardiac myocytes, MLK7 activated SAPK/JNK1, and ERK and p38 to a lesser extent. Additionally, MLK7 altered fetal gene expression and increased protein synthesis in cardiac myocytes. These data suggest that MLK7 is a new member of the mixed lineage kinase family that modulates cardiac SAPK/JNK pathway and may play a role in cardiac hypertrophy and progression to heart failure.

Cep-1347 (KT7515), a semisynthetic inhibitor of the mixed lineage kinase family

CEP-1347 (KT7515) promotes neuronal survival at dosages that inhibit activation of the c-Jun amino-terminal kinases (JNKs) in primary embryonic cultures and differentiated PC12 cells after trophic withdrawal and in mice treated with 1-methyl-4-phenyl tetrahydropyridine. In an effort to identify molecular target(s) of CEP-1347 in the JNK cascade, JNK1 and known upstream regulators of JNK1 were co-expressed in Cos-7 cells to determine whether CEP-1347 could modulate JNK1 activation. CEP-1347 blocked JNK1 activation induced by members of the mixed lineage kinase (MLK) family (MLK3, MLK2, MLK1, dual leucine zipper kinase, and leucine zipper kinase). The response was selective because CEP-1347 did not inhibit JNK1 activation in cells induced by kinases independent of the MLK cascade. CEP-1347 inhibition of recombinant MLK members in vitro was competitive with ATP, resulting in IC(50) values ranging from 23 to 51 nm, comparable to inhibitory potencies observed in intact cells. In addition, overexpression of MLK3 led to death in Chinese hamster ovary cells, and CEP-1347 blocked this death at doses comparable to those that inhibited MLK3 kinase activity. These results identify MLKs as targets of CEP-1347 in the JNK signaling cascade and demonstrate that CEP-1347 can block MLK-induced cell death.

The MLK family mediates c-Jun N-terminal kinase activation in neuronal apoptosis

Neuronal apoptotic death induced by nerve growth factor (NGF) deprivation is reported to be in part mediated through a pathway that includes Rac1 and Cdc42, mitogen-activated protein kinase kinases 4 and 7 (MKK4 and -7), c-Jun N-terminal kinases (JNKs), and c-Jun. However, additional components of the pathway remain to be defined. We show here that members of the mixed-lineage kinase (MLK) family (including MLK1, MLK2, MLK3, and dual leucine zipper kinase [DLK]) are expressed in neuronal cells and are likely to act between Rac1/Cdc42 and MKK4 and -7 in death signaling. Overexpression of MLKs effectively induces apoptotic death of cultured neuronal PC12 cells and sympathetic neurons, while expression of dominant-negative forms of MLKs suppresses death evoked by NGF deprivation or expression of activated forms of Rac1 and Cdc42. CEP-1347 (KT7515), which blocks neuronal death caused by NGF deprivation and a variety of additional apoptotic stimuli and which selectively inhibits the activities of MLKs, effectively protects neuronal PC12 cells from death induced by overexpression of MLK family members. In addition, NGF deprivation or UV irradiation leads to an increase in both level and phosphorylation of endogenous DLK. These observations support a role for MLKs in the neuronal death mechanism. With respect to ordering the death pathway, dominant-negative forms of MKK4 and -7 and c-Jun are protective against death induced by MLK overexpression, placing MLKs upstream of these kinases. Additional findings place the MLKs upstream of mitochondrial cytochrome c release and caspase activation.

Activated JNK phosphorylates the c-terminal domain of MLK2 that is required for MLK2-induced apoptosis

MAP kinase signaling pathways are important mediators of cellular responses to a wide variety of stimuli. Signals pass along these pathways via kinase cascades in which three protein kinases are sequentially phosphorylated and activated, initiating a range of cellular programs including cellular proliferation, immune and inflammatory responses, and apoptosis. One such cascade involves the mixed lineage kinase, MLK2, signaling through MAP kinase kinase 4 and/or MAP kinase kinase 7 to the SAPK/JNK, resulting in phosphorylation of transcription factors including the oncogene, c-jun. Recently we showed that MLK2 causes apoptosis in cultured neuronal cells and that this effect is dependent on activation of the JNK pathway (Liu, Y. F., Dorow, D. S., and Marshall, J. (2000) J. Biol. Chem. 275, 19035-19040). Furthermore, dominant-negative MLK2 blocked apoptosis induced by polyglutamine-expanded huntingtin protein, the product of the mutant Huntington's disease gene. Here we show that as well as activating the stress-signaling pathway, MLK2 is a target for phosphorylation by activated JNK. Phosphopeptide mapping of MLK2 proteins revealed that activated JNK2 phosphorylates multiple sites mainly within the noncatalytic C-terminal region of MLK2 including the C-terminal 100 amino acid peptide. In addition, MLK2 is phosphorylated in vivo within several of the same C-terminal peptides phosphorylated by JNK2 in vitro, and this phosphorylation is increased by cotransfection of JNK2 and treatment with the JNK activator, anisomycin. Cotransfection of dominant-negative JNK kinase inhibits phosphorylation of kinase-negative MLK2 by anisomycin-activated JNK. Furthermore, we show that the N-terminal region of MLK2 is sufficient to activate JNK but that removal of the C-terminal domain abrogates the apoptotic response. Taken together, these data indicate that the apoptotic activity of MLK2 is dependent on the C-terminal domain that is the main target for MLK2 phosphorylation by activated JNK.

The kinase activation loop is the key to mixed lineage kinase-3 activation via both autophosphorylation and hematopoietic progenitor kinase 1 phosphorylation

We have demonstrated previously that Cdc42 induced MLK-3 homodimerization leads to both autophosphorylation and activation of MLK-3 and postulated that autophosphorylation is an intermediate step of MLK-3 activation following its dimerization. In this report we sought to refine further the mechanism of MLK-3 activation and study the role of the putative kinase activation loop in MLK-3 activation. First we mutated the three potential phosphorylation sites in MLK-3 putative activation loop to alanine in an effort to abrogate MLK-3 autophosphorylation. Mutant T277A displayed almost no autophosphorylation activity and was nearly nonfunctional; mutant S281A, that displayed a low level of autophosphorylation, only slightly activated its downstream targets, whereas the T278A mutant, that exhibited autophosphorylation comparable to that of the wild type, was almost fully functional. Thus, these residues within the activation loop are critical for MLK-3 autophosphorylation and activation. In addition, when the Thr277 and Ser281 residues were mutated to negatively charged glutamic acid to mimic phosphorylated serine/threonine residues, the resulting mutants were fully functional, implying that these two residues may serve as the autophosphorylation sites. Interestingly, HPK1 also phosphorylated MLK-3 activation loop in vitro, and Ser281 was found to be the major phosphorylation site, indicating that HPK1 also activates MLK-3 via phosphorylation of the kinase activation loop.

Identification and characterization of functional domains in a mixed lineage kinase LZK

The mixed lineage kinase (MLK) family is a recently described protein kinase family. The MLKs contain a kinase domain followed by a dual leucine zipper-like motif. We previously reported the molecular cloning of LZK (leucine zipper-bearing kinase), a novel MLK, and that LZK activated the c-Jun NH2 terminal kinase (JNK)/stress-activated protein kinase (SAPK) pathway through MKK7 in cells. Here, we reveal that LZK forms dimers/oligomers through its dual leucine zipper-like motif, and that this is necessary for activation of the JNK/SAPK pathway. We also identify the C-terminal functional region of LZK, which is indispensable for the activation of SEK1, but not that of MKK7.

The mixed lineage kinase DLK is oligomerized by tissue transglutaminase during apoptosis

Current evidence suggests that the mixed lineage kinase family member dual leucine zipper-bearing kinase (DLK) might play a significant role in the regulation of cell growth and differentiation, particularly during the process of tissue remodeling. To further explore this working model, we have investigated the regulation of host and recombinant DLK in NIH3T3 and COS-1 cells undergoing apoptosis. Using calphostin C, a potent and selective inhibitor of protein kinase C and a recognized apoptosis inducer for various cell types, we demonstrate, by immunoblot analysis, that DLK protein levels are rapidly and dramatically down-regulated during the early phases of apoptosis. Down-regulation in calphostin C-treated cells was also accompanied by the appearance of SDS- and mercaptoethanol-resistant high molecular weight DLK immunoreactive oligomers. Experiments aimed at elucidating the mechanism(s) underlying DLK oligomerization revealed that the tissue transglutaminase (tTG) inhibitor monodansylcadaverine antagonized the effects of calphostin C almost completely, thereby suggesting the involvement of a tTG-catalyzed reaction as the root cause of DLK down-regulation and accumulation as high molecular weight species. In support of this notion, we also show that DLK can serve as a substrate for tTG-dependent cross-linking in vitro and that this covalent post-translational modification leads to the functional inactivation of DLK. Taken together, these observations suggest that transglutamination and oligomerization may constitute a relevant physiological mechanism for the regulation of DLK activity.

Zipper-mediated oligomerization of the mixed lineage kinase SPRK/MLK-3 is not required for its activation by the GTPase cdc 42 but Is necessary for its activation of the JNK pathway. Monomeric SPRK L410P does not catalyze the activating phosphorylation of Thr258 of murine MITOGEN-ACTIVATED protein kinase kinase 4

Src homology 3 domain-containing proline-rich kinase (SPRK)/mixed lineage kinase-3 is a serine/threonine kinase that has been identified as an upstream activator of the c-Jun NH(2)-terminal kinase (JNK) pathway. SPRK is capable of activating MKK4 by phosphorylation of serine and threonine residues, and mutant forms of MKK4 that lack the phosphorylation sites Ser(254) and Thr(258) block SPRK-induced JNK activation. A region of 63 amino acids following the kinase domain of SPRK is predicted to form a leucine zipper. The leucine zipper domain of SPRK has been shown to be necessary and sufficient for SPRK oligomerization, but its role in regulating activation of SPRK and downstream signaling remains unclear. In this study, we substituted a proposed stabilizing leucine residue in the zipper domain with a helix-disrupting proline to abrogate zipper-mediated SPRK oligomerization. We demonstrate that constitutively activated Cdc42 fully activates this monomeric SPRK mutant in terms of both autophosphorylation and histone phosphorylation activity and induces the same in vivo phosphorylation pattern as wild type SPRK. However, this catalytically active SPRK zipper mutant is unable to activate JNK. Our data show that the monomeric SPRK mutant fails to phosphorylate one of the two activating phosphorylation sites, Thr(258), of MKK4. These studies suggest that zipper-mediated SPRK oligomerization is not required for SPRK activation by Cdc42 but instead is critical for proper interaction and phosphorylation of a downstream target, MKK4.

Activation of MLK2-mediated signaling cascades by polyglutamine-expanded huntingtin

We previously reported that expression of polyglutamine-expanded huntingtin induces apoptosis via c-Jun amino-terminal kinase (JNK) activation in HN33 cells (Liu, Y. F. (1998) J. Biol. Chem. 273, 28873-28822). Extending this study, we now demonstrate a role of mixed-lineage kinase 2 (MLK2), a JNK activator, in polyglutamine-expanded huntingtin-mediated neuronal toxicity. We find that normal huntingtin interacts with MLK2, whereas the polyglutamine expansion interferes with this interaction. Similar to the expression of polyglutamine-expanded huntingtin, expression of MLK2 also induces JNK activation and apoptosis in HN33 cells. Co-expression of dominant negative MLK2 significantly attenuates neuronal apoptosis induced by the mutated huntingtin. Furthermore, over-expression of the N terminus of normal huntingtin partially rescues the neuronal toxicity induced by MLK2. Our results suggest that activation of MLK2-mediated signaling cascades may be partially involved in neuronal death induced by polyglutamine-expanded huntingtin.

Mixed-lineage kinase 3 delivers CD3/CD28-derived signals into the IkappaB kinase complex

The phosphorylation of IkappaB by the multiprotein IkappaB kinase complex (IKC) precedes the activation of transcription factor NF-kappaB, a key regulator of the inflammatory response. Here we identified the mixed-lineage group kinase 3 (MLK3) as an activator of NF-kappaB. Expression of the wild-type form of this mitogen-activated protein kinase kinase kinase (MAPKKK) induced nuclear immigration, DNA binding, and transcriptional activity of NF-kappaB. MLK3 directly phosphorylated and thus activated IkappaB kinase alpha (IKKalpha) and IKKbeta, revealing its function as an IkappaB kinase kinase (IKKK). MLK3 cooperated with the other two IKKKs, MEKK1 and NF-kappaB-inducing kinase, in the induction of IKK activity. MLK3 bound to components of the IKC in vivo. This protein-protein interaction was dependent on the central leucine zipper region of MLK3. A kinase-deficient version of MLK3 strongly impaired NF-kappaB-dependent transcription induced by T-cell costimulation but not in response to tumor necrosis factor alpha or interleukin-1. Accordingly, endogenous MLK3 was phosphorylated and activated by T-cell costimulation but not by treatment of cells with tumor necrosis factor alpha or interleukin-1. A dominant negative version of MLK3 inhibited NF-kappaB- and CD28RE/AP-dependent transcription elicited by the Rho family GTPases Rac and Cdc42, thereby providing a novel link between these GTPases and the IKC.

From receptors to stress-activated MAP kinases

The cell signaling pathways that culminate in activation of a family of stress-activated MAP kinases are beginning to be defined. Determination of cell life and cell death is known to largely depend on the balance of intrinsic life and death signals within cells. Recently, two representative mammalian stress-activated kinases, the JNK and p38 MAP kinases, have been implicated in determination of cell fate by modifying the life, death and differentiation signals. However, the molecular mechanisms by which extracellular signals are transmitted from membrane receptors to the most upstream kinases in the JNK and p38 signaling modules are not fully understood. This review will provide an overview of current knowledge of molecular links between inflammatory cytokine receptors and stress-activated MAP kinase cascades.

Isolation of the protein kinase TAO2 and identification of its mitogen-activated protein kinase/extracellular signal-regulated kinase kinase binding domain

We previously reported the cloning of the thousand and one-amino acid protein kinase 1 (TAO1), a rat homolog of the Saccharomyces cerevisiae protein kinase sterile 20 protein. Here we report the complete sequence and properties of a related rat protein kinase TAO2. Like TAO1, recombinant TAO2 selectively activated mitogen-activated protein/extracellular signal-regulated kinase kinases (MEKs) 3, 4, and 6 of the stress-responsive mitogen-activated protein kinase pathways in vitro and copurified with MEK3 endogenous to Sf9 cells. To examine TAO2 interactions with MEKs, the MEK binding domain of TAO2 was localized to an approximately 135-residue sequence just C-terminal to the TAO2 catalytic domain. In vitro this MEK binding domain associated with MEKs 3 and 6 but not MEKs 1, 2, or 4. Using chimeric MEK proteins, we found that the MEK N terminus was sufficient for binding to TAO2. Catalytic activity of full-length TAO2 enhanced its binding to MEKs. However, neither the autophosphorylation of the MEK binding domain of TAO2 nor the activity of MEK itself was required for MEK binding. These results suggest that TAO proteins lie in stress-sensitive kinase cascades and define a mechanism by which these kinases may organize downstream targets.

Localization of the mixed-lineage kinase DLK/MUK/ZPK to the Golgi apparatus in NIH 3T3 cells

DLK/MUK/ZPK is a serine/threonine kinase that belongs to the mixed-lineage (MLK) subfamily of protein kinases. As is the case for most members of this family, relatively little is known about the physiological role of DLK/MUK/ZPK in mammalian cells. Because analysis of subcellular distribution may provide important clues concerning the potential in vivo function of a protein, an antiserum was generated against the amino terminal region of murine DLK/MUK/ZPK and used for localization studies in wild-type NIH 3T3 cells. Light microscopic immunocytochemistry experiments performed with the antiserum revealed that DLK/MUK/ZPK was specifically localized in a juxtanuclear structure characteristic of the Golgi complex. In support of this, treatment of cells with brefeldin A, a drug known to disintegrate the Golgi apparatus, caused disruption of DLK/MUK/ZPK perinuclear staining. Ultrastructural observation of NIH 3T3 cells also confirmed this localization, showing that most of the immunoreactivity was detected on membranes of the stacked Golgi cisternae. Consistent with localization studies, biochemical analyses revealed that DLK/MUK/ZPK was predominantly associated with Golgi membranes on fractionation of cellular extracts and was entirely partitioned into the aqueous phase when membranes were subjected to Triton X-114 extraction. On the basis of these findings, we suggest that DLK/MUK/ZPK is a peripheral membrane protein tightly associated with the cytoplasmic face of the Golgi apparatus. (J Histochem Cytochem 47:1287-1296, 1999)

Expression of mixed lineage kinase 2 in germ cells of the testis

Mixed Lineage Kinase 2 is a mammalian protein kinase that activates stress-activated protein kinases/c-jun N-terminal kinases (SAPK/JNKs) through direct phosphorylation of their upstream activator, SEK1/JNKK. We have examined expression of both MLK2 and SEK1/JNKK RNAs in the rat testis at various times during postnatal development and in isolated testicular cell populations. We also have used immunohistochemistry to examine MLK2 protein expression and localization in adult rat and mouse testis. In these analyses, we found rat MLK2 mRNA expression was first evident at a very low level on day 25 after birth and present from day 35 at much higher levels that continue into adulthood. In RNA from isolated cell types, a MLK2 transcript was detected in primary spermatocytes and round spermatids, but not in Leydig or Sertoli cells. MLK2 RNA was also absent from the testis of rats after induced cryptorchidism. SEK1/JNKK transcripts, on the other hand, were present at all stages of testicular development and in all cell types tested. In tissue sections from both adult rat and mouse testis, MLK2 immunoreactivity was present in the nucleus of primary and secondary spermatocytes and round spermatids within seminiferous tubules, but was absent from spermatogonia. These findings indicate the JNK pathway is most likely ubiquitous in rodent testicular cells, while the cell-specific pattern of MLK2 expression suggests that it may be involved in the regulation of processes specific to post-mitotic germ cells. Furthermore, the finding of MLK2 protein in the nucleus of spermatocytes and round spermatids indicates a role for MLK2 in regulation of nuclear events specific to germ cell development.

Mitogen-activated protein kinase: conservation of a three-kinase module from yeast to human

Mitogen-activated protein kinases (MAPK) are serine-threonine protein kinases that are activated by diverse stimuli ranging from cytokines, growth factors, neurotransmitters, hormones, cellular stress, and cell adherence. Mitogen-activated protein kinases are expressed in all eukaryotic cells. The basic assembly of MAPK pathways is a three-component module conserved from yeast to humans. The MAPK module includes three kinases that establish a sequential activation pathway comprising a MAPK kinase kinase (MKKK), MAPK kinase (MKK), and MAPK. Currently, there have been 14 MKKK, 7 MKK, and 12 MAPK identified in mammalian cells. The mammalian MAPK can be subdivided into five families: MAPKerk1/2, MAPKp38, MAPKjnk, MAPKerk3/4, and MAPKerk5. Each MAPK family has distinct biological functions. In Saccharomyces cerevisiae, there are five MAPK pathways involved in mating, cell wall remodelling, nutrient deprivation, and responses to stress stimuli such as osmolarity changes. Component members of the yeast pathways have conserved counterparts in mammalian cells. The number of different MKKK in MAPK modules allows for the diversity of inputs capable of activating MAPK pathways. In this review, we define all known MAPK module kinases from yeast to humans, what is known about their regulation, defined MAPK substrates, and the function of MAPK in cell physiology.

Dimerization via tandem leucine zippers is essential for the activation of the mitogen-activated protein kinase kinase kinase, MLK-3

Mixed lineage kinase-3 (MLK-3) is a mitogen-activated kinase kinase kinase that mediates stress-activating protein kinase (SAPK)/c-Jun NH2-terminal kinase activation. MLK-3 and other MLK family kinases are characterized by the presence of multiple protein-protein interaction domains including a tandem leucine/isoleucine zipper (LZs) motif. Leucine zippers are known to mediate protein dimerization raising the possibility that the tandem leucine/isoleucine zippers may function as a dimerization motif of MLK-3. Using both co-immunoprecipitation and nonreducing SDS-polyacrylamide gel electrophoresis, we demonstrated that MLK-3 forms disulfide bridged homo-dimers and that the LZs motif is sufficient for MLK-3 homodimerization. We next asked whether MLK-3 utilizes a dimerization-based activation mechanism analogous to that of receptor tyrosine kinases. We found that dimerization via the LZs motif is a prerequisite for MLK-3 autophosphorylation. We then demonstrated that co-expression of Cdc42 lead to a substantial increase in MLK-3 dimerization, indicating that binding by this GTPase may induce MLK-3 dimerization. Moreover, the LZs minus form of MLK-3 failed to activate the downstream target SAPK, and expression of a MLK-3 LZs polypeptide was found to block SAPK activation by wild type MLK-3. Taken together, these findings indicate that dimerization plays a pivotal role in MLK-3 activation.

Isolation of TAO1, a protein kinase that activates MEKs in stress-activated protein kinase cascades

Several components of the budding yeast pheromone-response pathway are conserved in mammalian mitogen-activated protein (MAP) kinase pathways. Thus, we used degenerate oligonucleotides derived from the sequence of the Saccharomyces cerevisiae protein kinase Ste20p to amplify related sequences from the rat. One of these sequences was used to clone a rat Ste20p homolog, which we called TAO1 for its one thousand and one amino acids. Northern analysis shows TAO1 is highly expressed in brain, as is a homolog TAO2. Recombinant TAO1 was expressed and purified from Sf9 cells. In vitro, it activated MAP/extracellular signal-regulated protein kinase (ERK) kinases (MEKs) 3, 4, and 6 of the stress-responsive MAP kinase pathways, but not MEK1 or 2 of the classical MAP kinase pathway. TAO1 activated MEK3 but not MEK4 or MEK6 in transfected cells. MEK3 coimmunoprecipitated with TAO1 when they were expressed in 293 cells. In addition, immunoreactive MEK3 endogenous to Sf9 cells copurified with TAO1 produced from a recombinant baculovirus. The activation of and binding to MEK3 by TAO1 implicates TAO1 in the regulation of the p38-containing stress-responsive MAP kinase pathway.

Mixed-lineage kinase 2-SH3 domain binds dynamin and greatly enhances activation of GTPase by phospholipid

Mixed-lineage kinase 2 (MLK2) is a cytoplasmic protein kinase expressed at high levels in mammalian brain. The MLK2 structure is composed of a Src homology 3 (SH3) domain, two leucine zippers, a basic motif, a Cdc42/Rac interactive binding motif and a large C-terminal domain rich in proline, serine and threonine residues. To begin to define the role of MLK2 in mammalian brain, we used an MLK2-SH3 domain-glutathione S-transferase fusion protein (GST-MLK2-SH3) to isolate MLK2-binding proteins from rat brain extract. This analysis revealed that the major MLK2-SH3-domain-binding protein in rat brain is the GTPase dynamin. By using two different forms of the dynamin proline-rich domain as affinity ligands, the binding site for MLK2-SH3 was mapped to the C-terminal region of dynamin between residues 832 and 864. In GTPase assays, the addition of MLK2-SH3 stimulated the activity of purified dynamin I by 3-fold over the basal level, whereas the addition of a known dynamin activator, phosphatidylserine (PtdSer), stimulated a 6-fold increase. When MLK2-SH3 was added to the assay together with PtdSer, however, dynamin GTPase activity accelerated by more than 23-fold over basal level. An MLK2 mutant (MLK2-W59A-SH3), with alanine replacing a conserved tryptophan residue in the SH3 domain consensus motif, had no effect on dynamin activity, either alone or in the presence of PtdSer. In the same assay the SH3 domain from the regulatory subunit of phosphatidylinositol 3'-kinase stimulated a similar synergistic acceleration of dynamin GTPase activity in the presence of PtdSer. These results suggest that synergy between phospholipid and SH3 domain binding might be a general mechanism for the regulation of GTP hydrolysis by dynamin.

Zonal induction of mixed lineage kinase ZPK/DLK/MUK gene expression in regenerating mouse liver

ZPK/DLK/MUK is a serine/theronine kinase believed to be involved in the regulation of cell growth and differentiation. To further explore the suggested participation of ZPK/DLK/MUK in this process, we examined the expression and cellular localization of ZPK/DLK/MUK mRNA in regenerating mouse liver following partial hepatectomy by ribonuclease protection assay and in situ hybridization. The steady-state level of APK/DLKMUK mRNA was very low in normal and sham-operated mouse livers, whereas a marked and transient increase was observed in the regenerating liver. While ZPK/DLK/MUK mRNAs were rarely detected in hepatocytes from all zones of the normal liver, hepatocytes of regenerating liver exhibit a gradient of expression ranging from low in the periportal zone, to intermediate in the mid-zone, to high in the pericentral zone. These findings demonstrate a transient stimulation of ZPK/DLK/MUK gene expression that correlates with the growth response of hepatocyte subpopulations in regenerating liver.

Differential activation of stress-activated protein kinase kinases SKK4/MKK7 and SKK1/MKK4 by the mixed-lineage kinase-2 and mitogen-activated protein kinase kinase (MKK) kinase-1

Overexpression of the protein kinases mixed-lineage kinase-2 (MLK2) or mitogen-activated protein kinase (MAPK) kinase kinase-1 (MEKK1) is known to trigger the activation of stress-activated protein kinase (SAPK1)/c-Jun N-terminal kinase (JNK). Here we demonstrate that MLK2 activates SAPK kinase-1 (SKK1)/MAPK kinase 4 (MKK4) and SKK4/MKK7, the two known direct activators of SAPK1/JNK (both in transfection studies and in vitro). In contrast, MEKK1 activates SKK1/MKK4 more efficiently than MLK2, but barely activates SKK4/MKK7. Since SKK4/MKK7 (but not SKK1/MKK4) is activated by interleukin-1 and tumour necrosis factor in several cells and tissues, we suggest that MEKK1 does not mediate the activation of SKK4/MKK7 and SAPK1/JNK induced by these pro-inflammatory cytokines. MLK2 and MEKK1 also activated SKK2/MKK3 and SKK3/MKK6, the direct upstream activators of SAPK2a/p38.

Two-dimensional electrophoretic analysis of mixed lineage kinase 2 N-terminal domain binding proteins

The mixed lineage kinase 2 (MLK2) protein contains several structurally distinct domains including an src homology (SH) 3 domain, a kinase catalytic domain, two leucine zippers, a basic motif and a cdc42/rac interactive binding motif. These domains have been recognized mainly for their involvement in protein-protein interactions in signal transduction networks. The SH3 domain in particular has been implicated in control of signaling events. To identify proteins that interact with MLK2, the N-terminal 100 amino acids, including the SH3 domain, were expressed as a glutathione S-transferase (GST) fusion protein. This fusion protein (MLK2N) was used as an affinity ligand to isolate binding proteins from lysates of 35S-radiolabeled MDA-MB231 breast carcinoma cells. When the radiolabeled binding proteins were subjected to 2-DE, proteins of Mr 55,000, 31,500 and 34,000 bound consistently to the MLK2N domain fusion protein, but not to the GST control. Two of the binding proteins were isolated from whole cell lysates by preparative 2-DE and subjected to in-gel digestion and capillary or microbore reverse-phase high performance liquid chromatography (RP-HPLC). Resultant peptides were analyzed by peptide mass fingerprinting, N-terminal Edman degradation or tandem mass spectrometry. The 55,000 protein was identified as the cytoskeletal protein, beta-tubulin, and this was verified by immunoblotting of proteins in the MLK2N binding fraction with anti-tubulin antibodies. The 31,500 protein has been identified as prohibitin, a protein that has been implicated in both signal transduction and cell cycle arrest.

Two-dimensional gel database of human breast carcinoma cell expressed proteins: an update

Previously, we reported a two-dimensional gel map and database with molecular weight/isoelectric point (Mr/pI) loci for 22 proteins expressed in the breast carcinoma cell line, MDA-MB231 (Rasmussen et al., Electrophoresis 1997, 18, 588-598). Here we update this database with Mr/pI loci for a further nine cytoplasmic proteins and three Triton X-114 solubilised membrane proteins from MDA-MB231 cells. In addition, a novel protein, previously represented only in expressed sequence tag (EST) databases, has been identified as a Triton X-114 soluble protein and assigned an Mr/pI locus. During the course of isolating proteins from the Triton X-114 fraction, we compared recoveries of proteins in sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) gels after isoelectric focusing (IEF) using either immobilised pH gradients or carrier ampholytes. In these experiments, a significantly higher proportion of membrane proteins were visible in SDS-polyacrylamide gels after the use of carrier ampholytes for the first dimension. We also report our mass spectrometric-based procedure for identifying two-dimensional electrophoresis (2-DE) gel-resolved proteins, combining in-gel enzymatic digestion, 0.2 mm internal diameter (ID) capillary column reversed-phase high-performance liquid chromatography (RP-HPLC) peptide mapping and electrospray ionisation--ion trap--mass spectrometry.

Guanine-nucleotide exchange protein C3G activates JNK1 by a ras-independent mechanism. JNK1 activation inhibited by kinase negative forms of MLK3 and DLK mixed lineage kinases

Recently we have reported that the adaptor protein Crk transmits signals to c-Jun kinase (JNK) through C3G, a guanine-nucleotide exchange protein for the Ras family of small G proteins. Transient expression of C3G in 293T cells induced JNK1 activation without a significant effect on extracellular signal-related kinase 1 (ERK1), whereas mSos1 activated equally both JNK1 and ERK1. Coexpression of the dominant negative form of Ras-N17 did not suppress C3G-induced JNK1 activation but reduced the activity of JNK1 induced by mSos1, suggesting that Ras is not required for JNK activation by C3G. Ras-independent activation of JNK was supported by the finding that C3G-induced JNK activation was not inhibited by the dominant negative forms of Rac or Pak, which are components of the signaling pathway from Ras leading to JNK activation. In contrast, C3G-induced JNK1 activation was strongly inhibited by coexpression of the kinase negative forms of the mixed lineage kinase (MLK) family of proteins, MLK3 and dual leucine zipper kinase (DLK). In addition, MLK3-induced JNK1 activation was found to be suppressed by the kinase negative form of DLK, which bound to MLK3. These results suggest that C3G activates JNK1 through a pathway involving the MLK family of proteins.

Expression of mixed lineage kinase-1 in pancreatic beta-cell lines at different stages of maturation and during embryonic pancreas development

Events controlling differentiation to insulin-secreting beta-cells in the pancreas are not well understood, although beta-cells are thought to arise from pluripotent ductal precursor cells. To search for signaling proteins that might be involved in beta-cell maturation, we analyzed protein kinase expression in two developmentally and functionally distinct pancreatic beta-cell lines, RIN-5AH and RIN-A12, by reverse transcriptase polymerase chain reaction. A number of tyrosine and serine/threonine kinases were identified in both lines. One protein kinase, mixed lineage kinase-1 (MLK-1), was expressed at both the RNA and protein levels in RIN-5AH cells, which display an immature beta-cell phenotype, but was not detected in the more mature RIN-A12 cells. Furthermore, levels of MLK-1 mRNA and protein were increased after brief stimulation of RIN-5AH cells with either the differentiation inducer, sodium butyrate, or with serum after serum starvation. These increases in expression were independent of phenotypic markers such as insulin secretion or surface expression of major histocompatibility class I- and A2B5-reactive ganglioside. In addition, increases in MLK-1 expression in the stimulated RIN-5AH cells were accompanied by phosphorylation of MLK-1 on serine but not tyrosine. Antisense oligonucleotides to two distinct regions of MLK-1 caused RIN-5AH cells, but not RIN-A12 cells, to adopt a highly undifferentiated morphology, with a reduction in DNA synthesis and MLK-1 protein levels and elevated glucagon mRNA levels, but with no effect on insulin mRNA. In an immunohistochemical survey of embryonic mouse tissues, we found that temporal expression of MLK-1 was regulated in a tissue-specific manner. In the embryonic pancreas, MLK-1 expression was evident in ductal cells from day 13 to 16 but was not detected in late stage gestation or neonatal pancreas. These data suggest that MLK-1 is regulated in immature pancreatic beta-cells and their ductal precursors at the level of functional maturity and may therefore play a role in beta-cell development.

MST/MLK2, a member of the mixed lineage kinase family, directly phosphorylates and activates SEK1, an activator of c-Jun N-terminal kinase/stress-activated protein kinase

c-Jun N-terminal kinases/stress-activated protein kinases (JNKs/SAPKs) are mitogen-activated protein kinase (MAPK)-related protein kinases that are involved in several cellular events, including growth, differentiation, and apoptosis. Mixed lineage kinases (MLKs) form a family of protein kinases sharing two leucine zipper-like motifs and a kinase domain whose primary structure is similar to both the tyrosine-specific and the serine/threonine-specific kinase classes. We have reported that a member of the MLK family, MUK/DLK/ZPK, can activate JNK/SAPK in vivo, and here we show that another member of the MLK family, MST/MLK2, activates JNK/SAPK. Both MUK/DLK/ZPK and MST/MLK2 cause a slight activation of p38/Mpk2 when overexpressed in COS-1 cells, whereas MST/MLK2, but not MUK/DLK/ZPK, activates extracellular response kinase (ERK) to a certain degree. The activity of SEK1/MKK4/JNKK, a MAPK kinase class protein kinase designated as a direct activator of JNK/SAPK, is also induced by MUK/DLK/ZPK or MST/MLK2 overexpression. Furthermore, recombinant MST/MLK2 produced in bacteria directly phosphorylates and activates SEK1/MKK4/JNKK in vitro, showing that MST/MLK2 acts like a MAPK kinase kinase. Taken together, these results suggest that MLK family members are MAPK kinase kinases preferentially acting on the JNK/SAPK pathway.

Two-dimensional electrophoretic analysis of human breast carcinoma proteins: mapping of proteins that bind to the SH3 domain of mixed lineage kinase MLK2

MLK2, a member of the mixed lineage kinase (MLK) family of protein kinases, first reported by Dorow et al. (Eur. J. Biochem. 1993, 213, 701-710), comprises several distinct structural domains including an src homology-3 (SH3) domain, a kinase catalytic domain, a unique domain containing two leucine zipper motifs, a polybasic sequence, and a cdc42/rac interactive binding motif. Each of these domains has been shown in other systems to be associated with a specific type of protein interaction in the regulation of cellular signal transduction. To study the role of MLK2 in recruiting specific substrates, we constructed a recombinant cDNA encoding the N-terminal 100 amino acids of MLK2 (MLK2N), including the SH3 domain (residues 23-77), fused to glutathione S-transferase. This fusion protein was expressed in Escherichia coli, purified using gluthathione-Sepharose affinity chromatography and employed in an affinity approach to isolate MLK2-SH3 domain binding proteins from lysates of 35S-labelled MDA-MB231 human breast tumour cells. Electrophoretic analysis of bound proteins revealed that two low-abundance proteins with a molecular weights (Mr) of approximately 31,500 and approximately 34,000, bound consistently to the MLK2N protein. To establish accurately the Mt / isoelectric point (pI) loci of these MLK2-SH3 domain binding proteins, a number of abundant proteins in a two-dimensional electrophoresis (2-DE) master gel were identified to serve as triangulation marker points. Proteins were identified by (i) direct Edman degradation following electroblotting onto polyvinylidene difluoride (PVDF) membranes, (ii) Edman degradation of peptides generated by in-gel proteolysis and fractionation by rapid (approximately 12 min) microbore column (2.1 mm ID) reversed-phase high performance liquid chromatography (HPLC), (iii) mass spectrometric methods including peptide-mass fingerprinting and electrospray (ESI)-mass spectrometry (MS)-MS utilizing capillary (0.2-0.3 mm ID) column chromatography, or (iv) immunoblot analysis. Using this information, a preliminary 2-DE protein database for the human breast carcinoma cell line MDA-MB231, comprising 21 identified proteins, has been constructed and can be accessed via the World Wide Web (URL address: http:(/)/ www.ludwig.edu.au/www/jpsl/jpslhome.htm l).

Signal-transducing protein phosphorylation cascades mediated by Ras/Rho proteins in the mammalian cell: the potential for multiplex signalling

The features of three distinct protein phosphorylation cascades in mammalian cells are becoming clear. These signalling pathways link receptor-mediated events at the cell surface or intracellular perturbations such as DNA damage to changes in cytoskeletal structure, vesicle transport and altered transcription factor activity. The best known pathway, the Ras-->Raf-->MEK-->ERK cascade [where ERK is extracellular-signal-regulated kinase and MEK is mitogen-activated protein (MAP) kinase/ERK kinase], is typically stimulated strongly by mitogens and growth factors. The other two pathways, stimulated primarily by assorted cytokines, hormones and various forms of stress, predominantly utilize p21 proteins of the Rho family (Rho, Rac and CDC42), although Ras can also participate. Diagnostic of each pathway is the MAP kinase component, which is phosphorylated by a unique dual-specificity kinase on both tyrosine and threonine in one of three motifs (Thr-Glu-Tyr, Thr-Phe-Tyr or Thr-Gly-Tyr), depending upon the pathway. In addition to activating one or more protein phosphorylation cascades, the initiating stimulus may also mobilize a variety of other signalling molecules (e.g. protein kinase C isoforms, phospholipid kinases, G-protein alpha and beta gamma subunits, phospholipases, intracellular Ca2+). These various signals impact to a greater or lesser extent on multiple downstream effectors. Important concepts are that signal transmission often entails the targeted relocation of specific proteins in the cell, and the reversible formation of protein complexes by means of regulated protein phosphorylation. The signalling circuits may be completed by the phosphorylation of upstream effectors by downstream kinases, resulting in a modulation of the signal. Signalling is terminated and the components returned to the ground state largely by dephosphorylation. There is an indeterminant amount of cross-talk among the pathways, and many of the proteins in the pathways belong to families of closely related proteins. The potential for more than one signal to be conveyed down a pathway simultaneously (multiplex signalling) is discussed. The net effect of a given stimulus on the cell is the result of a complex intracellular integration of the intensity and duration of activation of the individual pathways. The specific outcome depends on the particular signalling molecules expressed by the target cells and on the dynamic balance among the pathways.