Cdc42 induces activation loop phosphorylation and membrane targeting of mixed lineage kinase 3

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.

Why Does Synergistic Activation of WASP, but Not N-WASP, by Cdc42 and PIP2 Require Cdc42 Prenylation?

Human WASP and N-WASP are homologous proteins that require the binding of multiple regulators, including the acidic lipid PIP2 and the small GTPase Cdc42, to relieve autoinhibition before they can stimulate the initiation of actin polymerization. Autoinhibition involves intramolecular binding of the C-terminal acidic and central motifs to an upstream basic region and GTPase binding domain. Little is known about how a single intrinsically disordered protein, WASP or N-WASP, binds multiple regulators to achieve full activation. Here we used molecular dynamics simulations to characterize the binding of WASP and N-WASP with PIP2 and Cdc42. In the absence of Cdc42, both WASP and N-WASP strongly associate with PIP2-containing membranes, through their basic region and also possibly through a tail portion of the N-terminal WH1 domain. The basic region also participates in Cdc42 binding, especially for WASP; consequently Cdc42 binding significantly compromises the ability of the basic region in WASP, but not N-WASP, to bind PIP2. PIP2 binding to the WASP basic region is restored only when Cdc42 is prenylated at the C-terminus and tethered to the membrane. This distinction in the activation of WASP and N-WASP may contribute to their different functional roles.

Negative Cross-Talk between TLR2/4-Independent AMPKα1 and TLR2/4-Dependent JNK Regulates S. pneumoniae-Induced Mucosal Innate Immune Response

Streptococcus pneumoniae is major cause of otitis media (OM) and life-threatening pneumonia. Overproduction of mucin, the major component of mucus, plays a critical role in the pathogenesis of both OM and pneumonia. However, the molecular mechanisms underlying the tight regulation of mucin upregulation in the mucosal epithelium by S. pneumoniae infection remain largely unknown. In this study, we show that S. pneumoniae pneumolysin (PLY) activates AMP-activated protein kinase α1 (AMPKα1), the master regulator of energy homeostasis, which is required for S. pneumoniae-induced mucin MUC5AC upregulation in vitro and in vivo. Moreover, we found that PLY activates AMPKα1 via cholesterol-dependent membrane binding of PLY and subsequent activation of the Ca2+- Ca2+/calmodulin-dependent kinase kinase β (CaMKKβ) and Cdc42-mixed-lineage protein kinase 3 (MLK3) signaling axis in a TLR2/4-independent manner. AMPKα1 positively regulates PLY-induced MUC5AC expression via negative cross-talk with TLR2/4-dependent activation of MAPK JNK, the negative regulator of MUC5AC expression. Moreover, pharmacological inhibition of AMPKα1 suppressed MUC5AC induction in the S. pneumoniae-induced OM mouse model, thereby demonstrating its therapeutic potential in suppressing mucus overproduction in OM. Taken together, our data unveil a novel mechanism by which negative cross-talk between TLR2/4-independent activation of AMPKα1 and TLR2/4-dependent activation of JNK tightly regulates the S. pneumoniae PLY-induced host mucosal innate immune response.

Mixed lineage kinase 3 requires a functional CRIB domain for regulation of blood pressure, cardiac hypertrophy, and left ventricular function

Mixed lineage kinase 3 (MLK3) modulates blood pressure and left ventricular function, but the mechanisms governing these effects remain unclear. In the current study, we therefore investigated the role of the MLK3 Cdc42/Rac interactive binding (CRIB) domain in cardiovascular physiology. We examined baseline and left ventricular pressure overload responses in a MLK3 CRIB mutant (MLK3C/C) mouse, which harbors point mutations in the CRIB domain to disrupt MLK3 activation by Cdc42. Male and female MLK3C/C mice displayed increased invasively measured blood pressure compared with wild-type (MLK3+/+) littermate controls. MLK3C/C mice of both sexes also developed left and right ventricular hypertrophy but normal baseline LV function by echocardiography and invasive hemodynamics. In LV tissue from MLK3C/C mice, map3k11 mRNA, which encodes MLK3, and MLK3 protein were reduced by 74 ± 6% and 73 ± 7%, respectively. After 1-wk LV pressure overload with 25-gauge transaortic constriction (TAC), male MLK3C/C mice developed no differences in LV hypertrophy but displayed reduction in the LV systolic indices ejection fraction and dP/dt normalized to instantaneous pressure. JNK activation was also reduced in LV tissue of MLK3C/C TAC mice. TAC induced MLK3 translocation from cytosolic fraction to membrane fraction in LV tissue from MLK3+/+ but not MLK3C/C mice. These findings identify a role of the MLK3 CRIB domain in MLK3 regulation of basal blood pressure and cardiac morphology, and in promoting the compensatory LV response to pressure overload.NEW & NOTEWORTHY Here, we identified that the presence of two discrete point mutations within the Cdc42/Rac interaction and binding domain of the protein MLK3 recapitulates the effects of whole body MLK3 deletion on blood pressure, cardiac hypertrophy, and left ventricular compensation after pressure overload. These findings implicate the CRIB domain, and thus MLK3 activation by this domain, as critical for maintenance of cardiovascular homeostasis.

Membrane repair triggered by cholesterol-dependent cytolysins is activated by mixed lineage kinases and MEK

Repair of plasma membranes damaged by bacterial pore-forming toxins, such as streptolysin O or perfringolysin O, during septic cardiomyopathy or necrotizing soft tissue infections is mediated by several protein families. However, the activation of these proteins downstream of ion influx is poorly understood. Here, we demonstrate that following membrane perforation by bacterial cholesterol-dependent cytolysins, calcium influx activates mixed lineage kinase 3 independently of protein kinase C or ceramide generation. Mixed lineage kinase 3 uncouples mitogen-activated kinase kinase (MEK) and extracellular-regulated kinase (ERK) signaling. MEK signals via an ERK-independent pathway to promote rapid annexin A2 membrane recruitment and enhance microvesicle shedding. This pathway accounted for 70% of all calcium ion-dependent repair responses to streptolysin O and perfringolysin O, but only 50% of repair to intermedilysin. We conclude that mixed lineage kinase signaling via MEK coordinates microvesicle shedding, which is critical for cellular survival against cholesterol-dependent cytolysins.

Cdc42/Rac Interactive Binding Containing Effector Proteins in Unicellular Protozoans With Reference to Human Host: Locks of the Rho Signaling

Small GTPases are the key to actin cytoskeleton signaling, which opens the lock of effector proteins to forward the signal downstream in several cellular pathways. Actin cytoskeleton assembly is associated with cell polarity, adhesion, movement and other functions in eukaryotic cells. Rho proteins, specifically Cdc42 and Rac, are the primary regulators of actin cytoskeleton dynamics in higher and lower eukaryotes. Effector proteins, present in an inactive state gets activated after binding to the GTP bound Cdc42/Rac to relay a signal downstream. Cdc42/Rac interactive binding (CRIB) motif is an essential conserved sequence found in effector proteins to interact with Cdc42 or Rac. A diverse range of Cdc42/Rac and their effector proteins have evolved from lower to higher eukaryotes. The present study has identified and further classified CRIB containing effector proteins in lower eukaryotes, focusing on parasitic protozoans causing neglected tropical diseases and taking human proteins as a reference point to the highest evolved organism in the evolutionary trait. Lower eukaryotes’ CRIB containing proteins fall into conventional effector molecules, PAKs (p21 activated kinase), Wiskoit-Aldrich Syndrome proteins family, and some have unique domain combinations unlike any known proteins. We also highlight the correlation between the effector protein isoforms and their selective specificity for Cdc42 or Rac proteins during evolution. Here, we report CRIB containing effector proteins; ten in Dictyostelium and Entamoeba, fourteen in Acanthamoeba, one in Trypanosoma and Giardia. CRIB containing effector proteins that have been studied so far in humans are potential candidates for drug targets in cancer, neurological disorders, and others. Conventional CRIB containing proteins from protozoan parasites remain largely elusive and our data provides their identification and classification for further in-depth functional validations. The tropical diseases caused by protozoan parasites lack combinatorial drug targets as effective paradigms. Targeting signaling mechanisms operative in these pathogens can provide greater molecules in combatting their infections.

c-Jun N-terminal kinase (JNK) signaling contributes to cystic burden in polycystic kidney disease

Polycystic kidney disease is an inherited degenerative disease in which the uriniferous tubules are replaced by expanding fluid-filled cysts that ultimately destroy organ function. Autosomal dominant polycystic kidney disease (ADPKD) is the most common form, afflicting approximately 1 in 1,000 people. It primarily is caused by mutations in the transmembrane proteins polycystin-1 (Pkd1) and polycystin-2 (Pkd2). The most proximal effects of Pkd mutations leading to cyst formation are not known, but pro-proliferative signaling must be involved for the tubule epithelial cells to increase in number over time. The c-Jun N-terminal kinase (JNK) pathway promotes proliferation and is activated in acute and chronic kidney diseases. Using a mouse model of cystic kidney disease caused by Pkd2 loss, we observe JNK activation in cystic kidneys and observe increased nuclear phospho c-Jun in cystic epithelium. Genetic removal of Jnk1 and Jnk2 suppresses the nuclear accumulation of phospho c-Jun, reduces proliferation and reduces the severity of cystic disease. While Jnk1 and Jnk2 are thought to have largely overlapping functions, we find that Jnk1 loss is nearly as effective as the double loss of Jnk1 and Jnk2. Jnk pathway inhibitors are in development for neurodegeneration, cancer, and fibrotic diseases. Our work suggests that the JNK pathway should be explored as a therapeutic target for ADPKD.

Autosomal dominant polycystic kidney disease is a leading cause of end stage renal disease requiring dialysis or kidney transplant. During disease development, the cells lining the kidney tubules proliferate. This proliferation transforms normally small diameter tubules into fluid-filled cysts that enlarge with time, eventually destroying all kidney function. Despite decades of research, polycystic kidney disease remains incurable. Furthermore, the precise signaling events involved in cyst initiation and growth remain unclear. The c-Jun N-terminal kinase (JNK), is a major pathway regulating cellular proliferation and differentiation but its importance to polycystic kidney disease was not known. We show that JNK activity is elevated in cystic kidneys and that reducing JNK activity decreases cyst growth pointing to JNK inhibition as a therapeutic strategy for treating polycystic kidney disease.

Protective Role of microRNA-31 in Acetaminophen-Induced Liver Injury: A Negative Regulator of c-Jun N-Terminal Kinase (JNK) Signaling Pathway

BACKGROUND & AIMS

Sustained c-Jun N-terminal kinase (JNK) activation plays a major role in drug-induced liver injury (DILI). Stress-responsive microRNA-31 (miR-31) has been implicated in regulating different cellular damage, and JNK activation could induce miR-31 expression. However, the regulatory role of miR-31 in DILI has not been studied previously. We aimed to investigate whether miR-31 could ameliorate DILI and ascertain potential molecular mechanism.

METHODS

miR-31 gene knockout (31-KO) and wild-type C57BL/6J mice were used to construct an acetaminophen (APAP)-induced DILI model. Primary mouse hepatocytes, as well as alpha mouse liver 12 (AML-12) cell lines, were used for in vitro experiments. Argonaute 2-associated RNA immunoprecipitation combined with high-throughput sequencing were performed to identify specific targets of miR-31.

RESULTS

31-KO mice showed a higher mortality rate, liver transaminase levels, and hepatic necrosis compared with those in wild-type mice after APAP-induced hepatotoxicity. The protective role of miR-31 on hepatocytes has been analyzed via constructing bone marrow chimeric mice. Mechanistically, we found that hepatic JNK phosphorylation increased significantly in 31-KO mice. This caused mitochondrial phosphorylated Src (p-Src) inactivation and more reactive oxygen species production, which directly amplifies hepatocyte necrotic cell death, while administration of JNK-specific inhibitor SP600125 could abrogate the differences. Moreover, bioinformatics analysis of RNA immunoprecipitation combined with high-throughput sequencing identified that guanosine triphosphatase, cell division cycle protein 42 (Cdc42), the upstream molecule of JNK signaling, was the specific target of miR-31 and could form a miR-31/Cdc42/phosphorylated mixed-lineage kinase 3 (p-MLK3) negative feedback loop to restrict JNK overactivation. Clinically, both miR-31 and phosphorylated JNK (p-JNK) were highly increased in liver tissues of DILI patients with different etiologies.

CONCLUSIONS

miR-31 can down-regulate Cdc42 to restrict overactivation of reactive oxygen species/JNK/mitochondria necrotic death loop in hepatocytes of APAP-induced DILI, which might provide a new therapeutic target for alleviating JNK overactivation-based liver injury.

LATS1 Regulates Mixed-Lineage Kinase 3 (MLK3) Subcellular Localization and MLK3-Mediated Invasion in Ovarian Epithelial Cells

Mixed-lineage kinase 3 (MLK3) activates mammalian mitogen-activated protein kinase (MAPK) signaling pathways in response to cytokines and stress stimuli. MLK3 is important for proliferation, migration, and invasion of different types of human tumor cells. We observed that endogenous MLK3 was localized to both the cytoplasm and the nucleus in immortalized ovarian epithelial (T80) and ovarian cancer cells, and mutation of arginines 474 and 475 within a putative MLK3 nuclear localization sequence (NLS) resulted in exclusion of MLK3 from the nucleus. The large tumor suppressor (LATS) Ser/Thr kinase regulates cell proliferation, morphology, apoptosis, and mitotic exit in response to cell-cell contact. RNA interference (RNAi)-mediated knockdown of LATS1 increased nuclear, endogenous MLK3 in T80 cells. LATS1 phosphorylated MLK3 on Thr477, which is within the putative NLS, and LATS1 expression enhanced the association between MLK3 and the adapter protein 14-3-3ζ. Thr477 is essential for MLK3-14-3-3ζ association and MLK3 retention in the cytoplasm, and a T477A MLK3 mutant had predominantly nuclear localization and significantly increased invasiveness of SKOV3 ovarian cancer cells. This study identified a novel link between the MAPK and Hippo/LATS1 signaling pathways. Our results reveal LATS1 as a novel regulator of MLK3 that controls MLK3 nuclear/cytoplasmic localization and MLK3-dependent ovarian cancer cell invasion.

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.

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.

JNK and cardiometabolic dysfunction

Cardiometabolic syndrome (CMS) describes the cluster of metabolic and cardiovascular diseases that are generally characterized by impaired glucose tolerance, intra-abdominal adiposity, dyslipidemia, and hypertension. CMS currently affects more than 25% of the world’s population and the rates of diseases are rapidly rising. These CMS conditions represent critical risk factors for cardiovascular diseases including atherosclerosis, heart failure, myocardial infarction, and peripheral artery disease (PAD). Therefore, it is imperative to elucidate the underlying signaling involved in disease onset and progression. The c-Jun N-terminal Kinases (JNKs) are a family of stress signaling kinases that have been recently indicated in CMS. The purpose of this review is to examine the in vivo implications of JNK as a potential therapeutic target for CMS. As the constellation of diseases associated with CMS are complex and involve multiple tissues and environmental triggers, carefully examining what is known about the JNK pathway will be important for specificity in treatment strategies.

Guanine nucleotide-binding protein G(i)α2 aggravates hepatic ischemia-reperfusion injury in mice by regulating MLK3 signaling

Hepatic ischemia-reperfusion (I/R) injury is a major challenge in liver resection and transplantation surgeries. Previous studies have revealed that guanine nucleotide-binding protein G(i)α2 (GNAI2) was involved in the progression of myocardial and cerebral I/R injury, but the role and function of GNAI2 in hepatic I/R have not been elucidated. The hepatocyte-specific GNAI2 knockout (GNAI2hep-/-) mice were generated and subjected to hepatic I/R injury. Primary hepatocytes isolated from GNAI2hep-/- and GNAI2flox/flox mice were cultured and challenged to hypoxia-reoxygenation insult. The specific function of GNAI2 in I/R-triggered hepatic injury and the underlying molecular mechanism were explored by various phenotypic analyses and molecular biology methods. In this study, we demonstrated that hepatic GNAI2 expression was significantly increased in liver transplantation patients and wild-type mice after hepatic I/R. Interestingly, hepatocyte-specific GNAI2 deficiency attenuated I/R-induced liver damage, inflammation cytokine expression, macrophage/neutrophil infiltration, and hepatocyte apoptosis in vivo and in vitro. Mechanistically, up-regulation of GNAI2 phosphorylates mixed-lineage protein kinase 3 (MLK3) through direct binding, which exacerbated hepatic I/R damage via MAPK and NF-κB pathway activation. Furthermore, blocking MLK3 signaling reversed GNAI2-mediated hepatic I/R injury. Our study firstly identifies GNAI2 as a promising target for prevention of hepatic I/R-induced injury and related liver diseases.-Sun, Q., He, Q., Xu, J., Liu, Q., Lu, Y., Zhang, Z., Xu, X., Sun, B. Guanine nucleotide-binding protein G(i)α2 aggravates hepatic ischemia-reperfusion injury in mice by regulating MLK3 signaling.

Mixed lineage kinase-3 prevents cardiac dysfunction and structural remodeling with pressure overload

Myocardial hypertrophy is an independent risk factor for heart failure (HF), yet the mechanisms underlying pathological cardiomyocyte growth are incompletely understood. The c-Jun NH2-terminal kinase (JNK) signaling cascade modulates cardiac hypertrophic remodeling, but the upstream factors regulating myocardial JNK activity remain unclear. In this study, we sought to identify JNK-activating molecules as novel regulators of cardiac remodeling in HF. We investigated mixed lineage kinase-3 (MLK3), a master regulator of upstream JNK-activating kinases, whose role in the remodeling process had not previously been studied. We observed increased MLK3 protein expression in myocardium from patients with nonischemic and hypertrophic cardiomyopathy and in hearts of mice subjected to transverse aortic constriction (TAC). Mice with genetic deletion of MLK3 (MLK3-/-) exhibited baseline cardiac hypertrophy with preserved cardiac function. MLK3-/- mice subjected to chronic left ventricular (LV) pressure overload (TAC, 4 wk) developed worsened cardiac dysfunction and increased LV chamber size compared with MLK3+/+ littermates ( n = 8). LV mass, pathological markers of hypertrophy ( Nppa, Nppb), and cardiomyocyte size were elevated in MLK3-/- TAC hearts. Phosphorylation of JNK, but not other MAPK pathways, was selectively impaired in MLK3-/- TAC hearts. In adult rat cardiomyocytes, pharmacological MLK3 kinase inhibition using URMC-099 blocked JNK phosphorylation induced by neurohormonal agents and oxidants. Sustained URMC-099 exposure induced cardiomyocyte hypertrophy. These data demonstrate that MLK3 prevents adverse cardiac remodeling in the setting of pressure overload. Mechanistically, MLK3 activates JNK, which in turn opposes cardiomyocyte hypertrophy. These results support modulation of MLK3 as a potential therapeutic approach in HF. NEW & NOTEWORTHY Here, we identified a role for mixed lineage kinase-3 (MLK3) as a novel antihypertrophic and antiremodeling molecule in response to cardiac pressure overload. MLK3 regulates phosphorylation of the stress-responsive JNK kinase in response to pressure overload and in cultured cardiomyocytes stimulated with hypertrophic agonists and oxidants. This study reveals MLK3-JNK signaling as a novel cardioprotective signaling axis in the setting of pressure overload.

Identification of two distinct peptide-binding pockets in the SH3 domain of human mixed-lineage kinase 3

Mixed-lineage kinase 3 (MLK3; also known as MAP3K11) is a Ser/Thr protein kinase widely expressed in normal and cancerous tissues, including brain, lung, liver, heart, and skeletal muscle tissues. Its Src homology 3 (SH3) domain has been implicated in MLK3 autoinhibition and interactions with other proteins, including those from viruses. The MLK3 SH3 domain contains a six-amino-acid insert corresponding to the n-Src insert, suggesting that MLK3 may bind additional peptides. Here, affinity selection of a phage-displayed combinatorial peptide library for MLK3's SH3 domain yielded a 13-mer peptide, designated "MLK3 SH3-interacting peptide" (MIP). Unlike most SH3 domain peptide ligands, MIP contained a single proline. The 1.2-Å crystal structure of the MIP-bound SH3 domain revealed that the peptide adopts a β-hairpin shape, and comparison with a 1.5-Å apo SH3 domain structure disclosed that the n-Src loop in SH3 undergoes an MIP-induced conformational change. A 1.5-Å structure of the MLK3 SH3 domain bound to a canonical proline-rich peptide from hepatitis C virus nonstructural 5A (NS5A) protein revealed that it and MIP bind the SH3 domain at two distinct sites, but biophysical analyses suggested that the two peptides compete with each other for SH3 binding. Moreover, SH3 domains of MLK1 and MLK4, but not MLK2, also bound MIP, suggesting that the MLK1-4 family may be differentially regulated through their SH3 domains. In summary, we have identified two distinct peptide-binding sites in the SH3 domain of MLK3, providing critical insights into mechanisms of ligand binding by the MLK family of kinases.

A combined linkage, microarray and exome analysis suggests MAP3K11 as a candidate gene for left ventricular hypertrophy

BACKGROUND

Electrocardiographic measures of left ventricular hypertrophy (LVH) are used as predictors of cardiovascular risk. We combined linkage and association analyses to discover novel rare genetic variants involved in three such measures and two principal components derived from them.

METHODS

The study was conducted among participants from the Erasmus Rucphen Family Study (ERF), a Dutch family-based sample from the southwestern Netherlands. Variance components linkage analyses were performed using Merlin. Regions of interest (LOD > 1.9) were fine-mapped using microarray and exome sequence data.

RESULTS

We observed one significant LOD score for the second principal component on chromosome 15 (LOD score = 3.01) and 12 suggestive LOD scores. Several loci contained variants identified in GWAS for these traits; however, these did not explain the linkage peaks, nor did other common variants. Exome sequence data identified two associated variants after multiple testing corrections were applied.

CONCLUSIONS

We did not find common SNPs explaining these linkage signals. Exome sequencing uncovered a relatively rare variant in MAPK3K11 on chromosome 11 (MAF = 0.01) that helped account for the suggestive linkage peak observed for the first principal component. Conditional analysis revealed a drop in LOD from 2.01 to 0.88 for MAP3K11, suggesting that this variant may partially explain the linkage signal at this chromosomal location. MAP3K11 is related to the JNK pathway and is a pro-apoptotic kinase that plays an important role in the induction of cardiomyocyte apoptosis in various pathologies, including LVH.

MLK3 phosphorylation by ERK1/2 is required for oxidative stress-induced invasion of colorectal cancer cells

Mixed lineage kinase 3 (MLK3) functions in migration and/or invasion of several human cancers; however, the role of MLK3 in colorectal cancer (CRC) invasion is unknown. MLK3 is a mitogen-activated protein kinase (MAPK) kinase kinase (MAP3K) which activates MAPK pathways through either kinase-dependent or -independent mechanisms. Human colorectal tumors display increased levels of reactive oxygen species (ROS) or oxidative stress. ROS, such as H2O2, are important for carcinogenesis and activate MAPK signaling pathways. In human colorectal carcinoma (HCT116) cells treated with H2O2, extracellular signal-regulated kinases 1 and 2 (ERK1/2) were activated and MLK3 exhibited reduced electrophoretic mobility (shift) in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), which was eliminated by phosphatase treatment. Pretreatment with the ROS scavenger N-acetyl-L-cysteine, the ERK1/2 inhibitor UO126, or ERK1/2 siRNA knockdown blocked the H2O2-induced shift of MLK3, while MLK3 inhibition with Cep1347 did not. In co-immunoprecipitation experiments performed on H2O2-treated HCT116 cells, endogenous MLK3 associated with endogenous ERK1/2 and B-Raf. Active ERK1 phosphorylated kinase dead FLAG-MLK3 in vitro, whereas ERK1 phosphorylation of kinase dead FLAG-MLK3-S705A-S758A was reduced. Both MLK3 siRNA knockdown and FLAG-MLK3-S705A-S758A expression decreased ERK1/2 activation in H2O2-treated cells. Prolonged H2O2 treatment activated ERK1/2 and promoted invasion of colon cancer cells, which was attenuated by MLK3 siRNA knockdown. Furthermore, S705A-S758A-FLAG-MLK3 demonstrated decreased oxidative-stress induced colon cancer cell invasion, but increased interaction with GST-B-Raf as compared with wild-type-FLAG-MLK3 in H2O2-treated cells. These results suggest oxidative stress stimulates an ERK1/2-dependent phosphorylation of MLK3 on Ser705 and Ser758, which promotes MLK3-dependent B-Raf and ERK1/2 activation; this positive feedback loop enhances the invasion of colon cancer cells.

EGFR Signals through a DOCK180-MLK3 Axis to Drive Glioblastoma Cell Invasion

A hallmark of glioblastoma (GBM) tumors is their highly invasive behavior. Tumor dissemination into surrounding brain tissue is responsible for incomplete surgical resection, and subsequent tumor recurrence. Identification of targets that control GBM cell dissemination is critical for developing effective therapies to treat GBM. A majority of GBM tumors have dysregulated EGFR signaling, due most frequently to EGFR amplification or the presence of a constitutively active EGFRvIII mutant. Mixed lineage kinase 3 (MLK3) is a mitogen-activated protein kinase kinase kinase (MAP3K) that can activate multiple MAPK pathways. In this study, evidence is provided that MLK3 is essential for GBM cell migration and invasion, and that an MLK inhibitor blocks EGF-induced migration and invasion. MLK3 silencing or MLK inhibition blocks EGF-induced JNK activation, suggesting that MLK3-JNK signaling promotes invasion of GBM cells. Mechanistically, it is demonstrated that DOCK180, a RAC1 guanine nucleotide exchange factor (GEF) overexpressed in invasive GBM cells, activates the MLK3-JNK signaling axis in a RAC1-dependent manner. In summary, this investigation identifies an EGFR-DOCK180-RAC1-MLK3-JNK signaling axis that drives glioblastoma cell migration and dissemination.Implications: On the basis of these findings, MLK3 emerges as a potential therapeutic target for the treatment of glioblastoma. Mol Cancer Res; 15(8); 1085-95. ©2017 AACR.

Heme oxygenase-1 protects spinal cord neurons from hydrogen peroxide-induced apoptosis via suppression of Cdc42/MLK3/MKK7/JNK3 signaling

The mechanisms by which oxidative stress induces spinal cord neuron death has not been completely understood. Investigation on the molecular signal pathways involved in oxidative stress-mediated neuronal death is important for development of new therapeutics for oxidative stress-associated spinal cord disorders. In current study we examined the role of heme oxygenase-1 (HO-1) in the modulation of MLK3/MKK7/JNK3 signaling, which is a pro-apoptotic pathway, after treating primary spinal cord neurons with H2O2. We found that MLK3/MKK7/JNK3 signaling was substantially activated by H2O2 in a time-dependent manner, demonstrated by increase of activating phosphorylation of MLK3, MKK7 and JNK3. H2O2 also induced expression of HO-1. Transduction of neurons with HO-1-expressing adeno-associated virus before H2O2 treatment introduced expression of exogenous HO-1 in neurons. Exogenous HO-1 reduced phosphorylation of MLK3, MKK7 and JNK3. Consistent with its inhibitory effect on MLK3/MKK7/JNK3 signaling, exogenous HO-1 decreased H2O2-induced neuronal apoptosis and necrosis. Furthermore, we found that exogenous HO-1 inhibited expression of Cdc42, which is crucial for MLK3 activation. In addition, HO-1-induced down-regulation of MLK3/MKK7/JNK3 signaling might be related to up-regulation of microRNA-137 (mir-137). A mir-137 inhibitor alleviated the inhibitory effect of HO-1 on JNK3 activation. This inhibitor also increased neuronal death even when exogenous HO-1 was expressed. Therefore, our study suggests a novel mechanism by which HO-1 exerted its neuroprotective efficacy on oxidative stress.

Heme Oxygenase-1 Inhibits Neuronal Apoptosis in Spinal Cord Injury through Down-Regulation of Cdc42-MLK3-MKK7-JNK3 Axis

The mechanism by which spinal cord injury (SCI) induces neuronal death has not been thoroughly understood. Investigation on the molecular signal pathways involved in SCI-mediated neuronal apoptosis is important for development of new therapeutics for SCI. In the current study, we explore the role of heme oxygenase-1 (HO-1) in the modulation of mixed lineage kinase 3/mitogen-activated protein kinase kinase/cJUN N-terminal kinase 3 (MLK3/MKK7/JNK3) signaling, which is a pro-apoptotic pathway, after SCI. We found that MLK3/MKK7/JNK3 signaling was activated by SCI in a time-dependent manner, demonstrated by increase in activating phosphorylation of MLK3, MKK7, and JNK3. SCI also induced HO-1 expression. Administration of HO-1-expressing adeno-associated virus before SCI introduced expression of exogenous HO-1 in injured spinal cords. Exogenous HO-1 reduced phosphorylation of MLK3, MKK7, and JNK3. Consistent with its inhibitory effect on MLK3/MKK7/JNK3 signaling, exogenous HO-1 decreased SCI-induced neuronal apoptosis and improved neurological score. Further, we found that exogenous HO-1 inhibited expression of cell division cycle 42 (Cdc42), which is crucial for MLK3 activation. In vitro experiments indicated that Cdc42 was essential for neuronal apoptosis, while transduction of neurons with HO-1-expressing adeno-associated virus significantly reduced neuronal apoptosis to enhance neuronal survival. Therefore, our study disclosed a novel mechanism by which HO-1 exerted its neuroprotective efficacy. Our discovery might be valuable for developing a new therapeutic approach for SCI.

JNK Signaling: Regulation and Functions Based on Complex Protein-Protein Partnerships

The c-Jun N-terminal kinases (JNKs), as members of the mitogen-activated protein kinase (MAPK) family, mediate eukaryotic cell responses to a wide range of abiotic and biotic stress insults. JNKs also regulate important physiological processes, including neuronal functions, immunological actions, and embryonic development, via their impact on gene expression, cytoskeletal protein dynamics, and cell death/survival pathways. Although the JNK pathway has been under study for >20 years, its complexity is still perplexing, with multiple protein partners of JNKs underlying the diversity of actions. Here we review the current knowledge of JNK structure and isoforms as well as the partnerships of JNKs with a range of intracellular proteins. Many of these proteins are direct substrates of the JNKs. We analyzed almost 100 of these target proteins in detail within a framework of their classification based on their regulation by JNKs. Examples of these JNK substrates include a diverse assortment of nuclear transcription factors (Jun, ATF2, Myc, Elk1), cytoplasmic proteins involved in cytoskeleton regulation (DCX, Tau, WDR62) or vesicular transport (JIP1, JIP3), cell membrane receptors (BMPR2), and mitochondrial proteins (Mcl1, Bim). In addition, because upstream signaling components impact JNK activity, we critically assessed the involvement of signaling scaffolds and the roles of feedback mechanisms in the JNK pathway. Despite a clarification of many regulatory events in JNK-dependent signaling during the past decade, many other structural and mechanistic insights are just beginning to be revealed. These advances open new opportunities to understand the role of JNK signaling in diverse physiological and pathophysiological states.

MLK3 Signaling in Cancer Invasion

Mixed-lineage kinase 3 (MLK3) was first cloned in 1994; however, only in the past decade has MLK3 become recognized as a player in oncogenic signaling. MLK3 is a mitogen-activated protein kinase kinase kinase (MAP3K) that mediates signals from several cell surface receptors including receptor tyrosine kinases (RTKs), chemokine receptors, and cytokine receptors. Once activated, MLK3 transduces signals to multiple downstream pathways, primarily to c-Jun terminal kinase (JNK) MAPK, as well as to extracellular-signal-regulated kinase (ERK) MAPK, P38 MAPK, and NF-κB, resulting in both transcriptional and post-translational regulation of multiple effector proteins. In several types of cancer, MLK3 signaling is implicated in promoting cell proliferation, as well as driving cell migration, invasion and metastasis.

JNK2 controls fragmentation of the Golgi complex and the G2/M transition through phosphorylation of GRASP65

In mammalian cells, the Golgi complex is composed of stacks that are connected by membranous tubules. During G2, the Golgi complex is disassembled into isolated stacks. This process is required for entry into mitosis, indicating that the correct inheritance of the organelle is monitored by a 'Golgi mitotic checkpoint'. However, the regulation and the molecular mechanisms underlying this Golgi disassembly are still poorly understood. Here, we show that JNK2 has a crucial role in the G2-specific separation of the Golgi stacks through phosphorylation of Ser277 of the Golgi-stacking protein GRASP65 (also known as GORASP1). Inhibition of JNK2 by RNA interference or by treatment with three unrelated JNK inhibitors causes a potent and persistent cell cycle block in G2. JNK activity becomes dispensable for mitotic entry if the Golgi complex is disassembled by brefeldin A treatment or by GRASP65 depletion. Finally, measurement of the Golgi fluorescence recovery after photobleaching demonstrates that JNK is required for the cleavage of the tubules connecting Golgi stacks. Our findings reveal that a JNK2-GRASP65 signalling axis has a crucial role in coupling Golgi inheritance and G2/M transition.

MLK3 phophorylates AMPK independently of LKB1

Emerging evidence has shown that cellular energy metabolism is regulated by the AMPK and MLK3-JNK signaling pathways, but the functional link between them remains to be determined. The present study aimed to explore the crosstalk between MLK3 and AMPK. We found that both JNK and AMPK were phosphorylated at their activation sites by TNF-α, Anisomycin, H2O2 and sorbitol. Interestingly, sorbitol stimulated phosphorylation of AMPK at T172 in LKB1-deficient cells. Following the screening of more than 100 kinases, we identified that MLK3 induced phosphorylation of AMPK at T172. Our in vitro analysis further revealed that MLK3-mediated phosphorylation of AMPK at T172 was independent of AMP, but addition of AMP caused a mobility shift of AMPK, an indication of autophosphorylation, suggesting that AMP binding and phosphorylation of T172 leads to maximal activation of AMPK. GST-pull down assays showed a direct interaction between AMPKα1 subunit and MLK3. Altogether, our results indicate that MLK3 serves as a common upstream kinase of AMPK and JNK and functions as a direct upstream kinase for AMPK independent of LKB1.

Inhibition of Cdc42-mediated activation of mixed lineage kinase 3 by the tumor suppressor protein merlin

Mammalian mitogen-activated protein kinase (MAPK) signaling pathways respond to diverse extracellular signals and coordinate a range of cellular responses. Mixed lineage kinase 3 (MLK3) is a member of the mixed lineage kinase family of MAPK kinase kinases (MAP3Ks) that functions to regulate multiple MAPK signaling pathways. Activated forms of the Rho GTP ases, Rac and Cdc42, interact with MLK3 through the Cdc42/Rac-interactive binding (CRIB) motif and promote MLK3 catalytic activity. Our recent findings demonstrate that merlin, the product of the neurofibromatosis type 2 (NF2) tumor suppressor gene, is a physiological inhibitor of MLK3. Our results suggest that merlin inhibits MLK3 activity by blocking the Cdc42-MLK3 interaction. In this commentary, the effect of merlin on Cdc42-mediated activation of MLK3 and MAPK signaling will be discussed.

The E3 ligase CHIP mediates ubiquitination and degradation of mixed-lineage kinase 3

Mixed-lineage kinase 3 (MLK3) activates mitogen-activated protein kinase (MAPK) signaling pathways and has important functions in migration, invasion, proliferation, tumorigenesis, and apoptosis. We investigated the role of the E3 ligase carboxyl terminus of Hsc70-interacting protein (CHIP) in the regulation of MLK3 protein levels. We show that CHIP interacts with MLK3 and, together with the E2 ubiquitin-conjugating enzyme UbcH5 (UbcH5a, -b, -c, or -d), ubiquitinates MLK3 in vitro. CHIP or Hsp70 overexpression promoted endogenous MLK3 ubiquitination and induced a decline in MLK3 protein levels in cells with Hsp90 inhibition. Furthermore, CHIP overexpression caused a proteasome-dependent reduction in exogenous MLK3 protein. Geldanamycin (GA), heat shock, and osmotic shock treatments also reduced the level of MLK3 protein via a CHIP-dependent mechanism. In addition, CHIP depletion in ovarian cancer SKOV3 cells increased cell invasion, and the enhancement of invasiveness was abrogated by small interfering RNA (siRNA)-mediated knockdown of MLK3. Thus, CHIP modulates MLK3 protein levels in response to GA and stress stimuli, and CHIP-dependent regulation of MLK3 is required for suppression of SKOV3 ovarian cancer cell invasion.

Cytoskeletal regulatory gene expression and migratory properties of B-cell progenitors are affected by the ETV6-RUNX1 rearrangement

Although the ETV6-RUNX1 fusion is a frequent initiating event in childhood leukemia, its role in leukemogenesis is only partly understood. The main impact of the fusion itself is to generate and sustain a clone of clinically silent preleukemic B-cell progenitors (BCP). Additional oncogenic hits, occurring even several years later, are required for overt disease. The understanding of the features and interactions of ETV6-RUNX1-positive cells during this "latency" period may explain how these silent cells can persist and whether they could be prone to additional genetic changes. In this study, two in vitro murine models were used to investigate whether ETV6-RUNX1 alters the cellular adhesion and migration properties of BCP. ETV6-RUNX1-expressing cells showed a significant defect in the chemotactic response to CXCL12, caused by a block in CXCR4 signaling, as demonstrated by inhibition of CXCL12-associated calcium flux and lack of ERK phosphorylation. Moreover, the induction of ETV6-RUNX1 caused changes in the expression of cell-surface adhesion molecules. The expression of genes regulating the cytoskeleton was also affected, resulting in a block of CDC42 signaling. The abnormalities described here could alter the interaction of ETV6-RUNX1 preleukemic BCP with the microenvironment and contribute to the pathogenesis of the disease.

IMPLICATIONS

Alterations in the expression of cytoskeletal regulatory genes and migration properties of BCP represent early events in the evolution of the disease, from the preleukemic phase to the clinical onset, and suggest new strategies for effective eradication of leukemia.

Domain specificity of MAP3K family members, MLK and Tak1, for JNK signaling in Drosophila

A highly diverse set of protein kinases functions as early responders in the mitogen- and stress-activated protein kinase (MAPK/SAPK) signaling pathways. For instance, humans possess 14 MAPK kinase kinases (MAP3Ks) that activate Jun kinase (JNK) signaling downstream. A major challenge is to decipher the selective and redundant functions of these upstream MAP3Ks. Taking advantage of the relative simplicity of Drosophila melanogaster as a model system, we assessed MAP3K signaling specificity in several JNK-dependent processes during development and stress response. Our approach was to generate molecular chimeras between two MAP3K family members, the mixed lineage kinase, Slpr, and the TGF-β activated kinase, Tak1, which share 32% amino acid identity across the kinase domain but otherwise differ in sequence and domain structure, and then test the contributions of various domains for protein localization, complementation of mutants, and activation of signaling. We found that overexpression of the wild-type kinases stimulated JNK signaling in alternate contexts, so cells were capable of responding to both MAP3Ks, but with distinct outcomes. Relative to wild-type, the catalytic domain swaps compensated weakly or not at all, despite having a shared substrate, the JNK kinase Hep. Tak1 C-terminal domain-containing constructs were inhibitory in Tak1 signaling contexts, including tumor necrosis factor-dependent cell death and innate immune signaling; however, depressing antimicrobial gene expression did not necessarily cause phenotypic susceptibility to infection. These same constructs were neutral in the context of Slpr-dependent developmental signaling, reflecting differential subcellular protein localization and by inference, point of activation. Altogether, our findings suggest that the selective deployment of a particular MAP3K can be attributed in part to its inherent sequence differences, cellular localization, and binding partner availability.

The small GTPase RALA controls c-Jun N-terminal kinase-mediated FOXO activation by regulation of a JIP1 scaffold complex

FOXO (forkhead box O) transcription factors are tumor suppressors and increase the life spans of model organisms. Cellular stress, in particular oxidative stress caused by an increase in levels of reactive oxygen species (ROS), activates FOXOs through JNK-mediated phosphorylation. Importantly, JNK regulation of FOXO is evolutionarily conserved. Here we identified the pathway that mediates ROS-induced JNK-dependent FOXO regulation. Following increased ROS, RALA is activated by the exchange factor RLF (RalGDS-like factor), which is in complex with JIP1 (C-Jun-amino-terminal-interacting protein 1) and JNK. Active RALA consequently regulates assembly and activation of MLK3, MKK4, and JNK onto the JIP1 scaffold. Furthermore, regulation of FOXO by RALA and JIP1 is conserved in C. elegans, where both ral-1 and jip-1 depletion impairs heat shock-induced nuclear translocation of the FOXO orthologue DAF16.

Involvement of mixed lineage kinase 3 in cancer

Mitogen-activated protein kinase (MAPK) signaling pathways are composed of a phosphorelay signaling module where an activated MAP kinase kinase kinase (MAP3K) phosphorylates and activates a MAPK kinase (MAP2K) that in turn phosphorylates and activates a MAPK. The biological outcome of MAPK signaling is the regulation of cellular responses such as proliferation, differentiation, migration, and apoptosis. The MAP3K mixed lineage kinase 3 (MLK3) phosphorylates MAP2Ks to activate multiple MAPK signaling pathways, and MLK3 also has functions in cell signaling that are independent of its kinase activity. The recent elucidation of essential functions for MLK3 in tumour cell proliferation, migration, and invasion has drawn attention to the MLKs as potential therapeutic targets for cancer treatments. The mounting evidence that suggests a role for MLK3 in tumourigenesis and establishment of the malignant phenotype is the focus of this review.

Lysine 63-linked ubiquitination modulates mixed lineage kinase-3 interaction with JIP1 scaffold protein in cytokine-induced pancreatic β cell death

The mixed lineage kinase MLK3 plays a crucial role in compromising mitochondrial integrity and functions as a proapoptotic competence factor in the early stages of cytokine-induced pancreatic β cell death. In an effort to identify mechanisms that regulate MLK3 activity in β cells, we discovered that IL-1β stimulates Lys-63-linked ubiquitination of MLK3 via a conserved, TRAF6-binding peptapeptide motif in the catalytic domain of the kinase. TRAF6-mediated ubiquitination was required for dissociation of inactive monomeric MLK3 from the scaffold protein IB1/JIP1, facilitating the subsequent dimerization, autophosphorylation, and catalytic activation of MLK3. Inability to ubiquitinate MLK3, or the presence of A20, an upstream Lys-63-linked deubiquitinase, strongly curtailed the ability of MLK3 to affect the proapoptotic translocation of BAX in cytokine-stimulated pancreatic β cells, an early step in the progression toward β cell death. These studies suggest a novel mechanism for MLK3 activation and provide new clues for therapeutic intervention in promoting β cell survival.

The MLK-related kinase (MRK) is a novel RhoC effector that mediates lysophosphatidic acid (LPA)-stimulated tumor cell invasion

The small GTPase RhoC is overexpressed in many invasive tumors and is essential for metastasis. Despite its high structural homology to RhoA, RhoC appears to perform functions that are different from those controlled by RhoA. The identity of the signaling components that are differentially regulated by these two GTPases is only beginning to emerge. Here, we show that the MAP3K protein MRK directly binds to the GTP-bound forms of both RhoA and RhoC in vitro. However, siRNA-mediated depletion of MRK in cells phenocopies depletion of RhoC, rather than that of RhoA. MRK depletion, like that of RhoC, inhibits LPA-stimulated cell invasion, while depletion of RhoA increases invasion. We also show that active MRK enhances LPA-stimulated invasion, further supporting a role for MRK in the regulation of invasion. Depletion of either RhoC or MRK causes sustained myosin light chain phosphorylation after LPA stimulation. In addition, activation of MRK causes a reduction in myosin light chain phosphorylation. In contrast, as expected, depletion of RhoA inhibits myosin light chain phosphorylation. We also present evidence that both RhoC and MRK are required for LPA-induced stimulation of the p38 and ERK MAP kinases. In conclusion, we have identified MRK as a novel RhoC effector that controls LPA-stimulated cell invasion at least in part by regulating myosin dynamics, ERK and p38.

MLK3 regulates paxillin phosphorylation in chemokine-mediated breast cancer cell migration and invasion to drive metastasis

MLK3 kinase activates multiple mitogen-activated protein kinases and plays a critical role in cancer cell migration and invasion. In the tumor microenvironment, prometastatic factors drive breast cancer invasion and metastasis, but their associated signaling pathways are not well-known. Here, we provide evidence that MLK3 is required for chemokine (CXCL12)-induced invasion of basal breast cancer cells. We found that MLK3 induced robust phosphorylation of the focal adhesion scaffold paxillin on Ser 178 and Tyr 118, which was blocked by silencing or inhibition of MLK3-JNK. Silencing or inhibition of MLK3, inhibition of JNK, or expression of paxillin S178A all led to enhanced Rho activity, indicating that the MLK3-JNK-paxillin axis limits Rho activity to promote focal adhesion turnover and migration. Consistent with this, MLK3 silencing increased focal adhesions and stress fibers in breast cancer cells. MLK3 silencing also decreased the formation of breast cancer lung metastases in vivo, and breast cancer cells derived from mouse lung metastases showed enhanced Ser 178 paxillin phosphorylation. Taken together, our findings suggest that the MLK3-JNK-paxillin signaling axis may represent a potential therapeutic target and/or prognostic marker in breast cancer metastasis.

Drosophila heat shock response requires the JNK pathway and phosphorylation of mixed lineage kinase at a conserved serine-proline motif

Defining context specific requirements for proteins and pathways is a major challenge in the study of signal transduction. For example, the stress-activated protein kinase (SAPK) pathways are comprised of families of closely related transducers that are activated in a variety of tissues and contexts during development and organismal homeostasis. Consequently, redundant and pleiotropic effects have hampered a complete understanding of the individual contributions of transducers in distinct contexts. Here, we report on the function of a context-specific regulatory phosphorylation site, PXSP, in the Drosophila mixed lineage kinase protein, Slpr, a mitogen-activated protein kinase kinase kinase (MAP3K) in the Jun Kinase (JNK) pathway. Genetic analysis of the function of non-phosphorylatable (PXAP) and phosphomimetic mutant (PXEP) Slpr transgenes in several distinct contexts revealed minimal effects in JNK-dependent tissue closure processes but differential requirements in heat stress response. In particular, PXAP expression resulted in sensitivity of adults to sustained heat shock, like p38 and JNK pathway mutants. In contrast, PXEP overexpression conferred some resistance. Indeed, phosphorylation of the PXSP motif is enriched under heat shock conditions and requires in part, the p38 kinases for the enrichment. These data suggest that coordination of signaling between p38 and Slpr serves to maintain JNK signaling during heat stress. In sum, we demonstrate a novel role for JNK signaling in the heat shock response in flies and identify a posttranslational modification on Slpr, at a conserved site among MAP3K mixed lineage kinase family members, which bolsters stress resistance with negligible effects on JNK-dependent developmental processes.

SH3 domains: modules of protein-protein interactions

Src homology 3 (SH3) domains are involved in the regulation of important cellular pathways, such as cell proliferation, migration and cytoskeletal modifications. Recognition of polyproline and a number of noncanonical sequences by SH3 domains has been extensively studied by crystallography, nuclear magnetic resonance and other methods. High-affinity peptides that bind SH3 domains are used in drug development as candidates for anticancer treatment. This review summarizes the latest achievements in deciphering structural determinants of SH3 function.

The Golgi in cell migration: regulation by signal transduction and its implications for cancer cell metastasis

Migration and invasion are fundamental features of metastatic cancer cells. The Golgi apparatus, an organelle involved in posttranslational modification and sorting of proteins, is widely accepted to regulate directional cell migration. In addition, mounting evidence suggests that the Golgi is a hub for different signaling pathways. In this paper we will give an overview on how polarized secretion and microtubule nucleation at the Golgi regulate directional cell migration. We will review different signaling pathways that signal to and from the Golgi. Finally, we will discuss how these signaling pathways regulate the role of the Golgi in cell migration and invasion. We propose that by identifying regulators of the Golgi, we might be able to uncover unappreciated modulators of cell migration. Uncovering the regulatory network that orchestrates cell migration is of fundamental importance for the development of new therapeutic strategies against cancer cell metastasis.

MLK4β functions as a negative regulator of MAPK signaling and cell invasion

Mixed lineage kinase (MLK) 4, or MLK4, is a member of the MLK family of mitogen-activated protein kinase kinase kinases (MAP3Ks). Typically, MAP3Ks function to activate the mitogen-activated protein kinase (MAPK)-signaling pathways and regulate different cellular responses. However, here we report that MLK4β, unlike the other MLKs, negatively regulates the activities of the MAPKs, p38, c-Jun N-terminal kinase and extracellular signal-regulated kinase, and the MAP2Ks, MEK3 and 6. Our results show that MLK4β inhibits sorbitol- and tumor necrosis factor-induced activation of p38. Furthermore, MLK4β interacts with another MLK family member, MLK3, in HCT116 cells. Exogenous expression of MLK4β inhibits activation of MLK3 and also blocks matrix metalloproteinase-9 gelatinase activity and invasion in SKOV3 ovarian cancer cells. Collectively, our data establish MLK4β as a novel suppressor of MLK3 activation, MAPK signaling and cell invasion.

MLK3 regulates bone development downstream of the faciogenital dysplasia protein FGD1 in mice

Mutations in human FYVE, RhoGEF, and PH domain-containing 1 (FGD1) cause faciogenital dysplasia (FGDY; also known as Aarskog syndrome), an X-linked disorder that affects multiple skeletal structures. FGD1 encodes a guanine nucleotide exchange factor (GEF) that specifically activates the Rho GTPase CDC42. However, the mechanisms by which mutations in FGD1 affect skeletal development are unknown. Here, we describe what we believe to be a novel signaling pathway in osteoblasts initiated by FGD1 that involves the MAP3K mixed-lineage kinase 3 (MLK3). We observed that MLK3 functions downstream of FGD1 to regulate ERK and p38 MAPK, which in turn phosphorylate and activate the master regulator of osteoblast differentiation, Runx2. Mutations in FGD1 found in individuals with FGDY ablated its ability to activate MLK3. Consistent with our description of this pathway and the phenotype of patients with FGD1 mutations, mice with a targeted deletion of Mlk3 displayed multiple skeletal defects, including dental abnormalities, deficient calvarial mineralization, and reduced bone mass. Furthermore, mice with knockin of a mutant Mlk3 allele that is resistant to activation by FGD1/CDC42 displayed similar skeletal defects, demonstrating that activation of MLK3 specifically by FGD1/CDC42 is important for skeletal mineralization. Thus, our results provide a putative biochemical mechanism for the skeletal defects in human FGDY and suggest that modulating MAPK signaling may benefit these patients.

Regulation of mixed lineage kinase 3 is required for Neurofibromatosis-2-mediated growth suppression in human cancer

The Neurofibromatosis-2 (NF2) tumor suppressor merlin negatively regulates cell proliferation in numerous cell types. We have previously shown that the NF2 protein (merlin/schwannomin) associates with mixed lineage kinase 3 (MLK3), a mitogen-activated protein kinase (MAPK) kinase kinase that is required for the proliferation of normal and neoplastic cells. In this study, we show that merlin inhibits MLK3 activity, as well as the activation of its downstream effectors, B-Raf, extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK). The ability of merlin to regulate MLK3 activity requires a direct association between MLK3 and residues in the C-terminal region of merlin. Merlin integrates Rho GTPase family signaling with MAPK activity by inhibiting the binding between MLK3 and its upstream activator, Cdc42. Furthermore, we demonstrate that MLK3 is required for merlin-mediated suppression of cell proliferation and invasion. Collectively, these results establish merlin as a potent inhibitor of MLK3, ERK and JNK activation in cancer, and provide a mechanistic link between deregulated MAPK and Rho GTPase signaling in NF2 growth control.

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.

Dyad of CD40/CD40 ligand fosters neuroinflammation at the blood-brain barrier and is regulated via JNK signaling: implications for HIV-1 encephalitis

Human immunodeficiency virus 1 (HIV-1) infection may result in activation of peripheral monocytes followed by their infiltration into the CNS, where the release of proinflammatory mediators causes neurologic disease. Previously, we detected high levels of soluble CD40 ligand (CD40L) in CSF and plasma of HIV-infected patients with cognitive impairment. We now show that CD40, a receptor for CD40L, is highly expressed in brain endothelial cells of patients affected by HIV-1 encephalitis (HIVE), suggesting an important role for the CD40/CD40L dyad in regulating blood-brain barrier (BBB) functions. This concept was further supported by in vitro experiments. Exposure of primary human brain microvascular endothelial cells (BMVECs) to CD40L upregulated the expression of adhesion molecules intracellular adhesion molecule-1 and vascular cell adhesion molecule-1, which caused a fourfold increase in monocyte adhesion to BMVECs and stimulated migration across an in vitro BBB model. Investigations into the intracellular signaling pathways that govern these events revealed that cJUN-N-terminal kinase (JNK) is critical to CD40 activation in the BMVECs. CD40L induced activation of mixed-lineage-kinase-3 and JNK, leading to the subsequent activation of cJUN/AP-1 (activating-protein-1). JNK inhibition in the BMVECs prevented CD40L-mediated induction of adhesion molecules, monocyte adhesion, and transendothelial migration. These new findings support the concept that the CD40/CD40L dyad plays an important role in HIVE neuroinflammation.

Induced, selective proteolysis of MLK3 negatively regulates MLK3/JNK signalling

MLK3 (mixed lineage kinase 3) is a MAP3K [MAPK (mitogen-activated protein kinase) kinase kinase] that activates multiple MAPK pathways, including the JNK (c-Jun N-terminal kinase) pathway. Immunoblotting of lysates from cells ectopically expressing active MLK3 revealed an additional immunoreactive band corresponding to a CTF (C-terminal fragment) of MLK3. In the present paper we provide evidence that MLK3 undergoes proteolysis to generate a stable CTF in response to different stimuli, including PMA and TNFalpha (tumour necrosis factor alpha). The cleavage site was deduced by Edman sequencing as between Gln251 and Pro252, which is within the kinase domain of MLK3. Based on our homology model of the kinase domain of MLK3, the region containing the cleavage site is predicted to reside on a flexible solvent-accessible loop. Site-directed mutagenesis studies revealed that Leu250 and Gln251 are required for recognition by the 'MLK3 protease', reminiscent of the substrate specificity of the coronavirus 3C and 3CL proteases. Whereas numerous mammalian protease inhibitors have no effect on MLK3 proteolysis, blockade of the proteasome through epoxomicin or MG132 abolishes PMA-induced production of the CTF of MLK3. This CTF is able to heterodimerize with full-length MLK3, and interact with the active form of the small GTPase Cdc42, resulting in diminished activation loop phosphorylation of MLK3 and reduced signalling to JNK. Thus this novel proteolytic processing of MLK3 may negatively control MLK3 signalling to JNK.

Evolution of domain combinations in protein kinases and its implications for functional diversity

Protein kinases phosphorylating Ser/Thr/Tyr residues in several cellular proteins exert tight control over their biological functions. They constitute the largest protein family in most eukaryotic species. Protein kinases classified based on sequence similarity in their catalytic domains, cluster into subfamilies, which share gross functional properties. Many protein kinases are associated or tethered covalently to domains that serve as adapter or regulatory modules, aiding substrate recruitment, specificity, and also serve as scaffolds. Hence the modular organisation of the protein kinases serves as guidelines to their functional and molecular properties. Analysis of genomic repertoires of protein kinases in eukaryotes have revealed wide spectrum of domain organisation across various subfamilies of kinases. Occurrence of organism-specific novel domain combinations suggests functional diversity achieved by protein kinases in order to regulate variety of biological processes. In addition, domain architecture of protein kinases revealed existence of hybrid protein kinase subfamilies and their emerging roles in the signaling of eukaryotic organisms. In this review we discuss the repertoire of non-kinase domains tethered to multi-domain kinases in the metazoans. Similarities and differences in the domain architectures of protein kinases in these organisms indicate conserved and unique features that are critical to functional specialization.

Dependence of leucine-rich repeat kinase 2 (LRRK2) kinase activity on dimerization

Dominant missense mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common known genetic cause of Parkinson disease. LRRK2 encodes a serine/threonine protein kinase, and pathogenic mutations may increase kinase activity. Intrinsic GTP binding in the GTPase domain may govern kinase activity through an internal signal transduction cascade. As with many protein kinases, LRRK2 self-interacts through mechanisms that may regulate enzymatic activity. We find that the disruption of either GTPase or kinase activity enhances the formation of high molecular weight oligomers and prevents the formation of LRRK2 dimer structures. In addition, brief application of the broad spectrum kinase inhibitor staurosporine ablates LRRK2 dimers and promotes LRRK2 high molecular weight oligomers. LRRK2 interactions with other proteins in cell lines are kinase-independent and include chaperones and cell cytoskeleton components, suggesting that LRRK2 self-assembly principally dictates complex size. To further explore the mechanics of kinase activation, we separate soluble LRRK2 protein that encodes the pathogenic G2019S mutation into high molecular weight oligomers, dimers, and monomers and find that kinase activity resides with dimeric LRRK2. Some PD-associated mutations that increase kinase activity in vitro significantly increase the proportion of dimer structures relative to total LRRK2 protein, providing additional insight into how pathogenic mutations may alter normal enzymatic regulation. Targeting and tracking LRRK2 dimerization may provide a clear way to observe LRRK2 kinase activity in living cells, and disruption of dimeric LRRK2 through kinase inhibition or other means may attenuate pathogenic increases in LRRK2 enzymatic output.

MLK3 limits activated Galphaq signaling to Rho by binding to p63RhoGEF

Mixed lineage kinase 3 (MLK3) is a MAP3K that activates the JNK-dependent MAPK pathways. Here, we show that MLK3 is required for cell migration in a manner independent of its role as a MAP3K or MLK3 kinase activity. Rather, MLK3 functions in a regulated way to limit levels of the activated GTPase Rho by binding to the Rho activator, p63RhoGEF/GEFT, which, in turn, prevents its activation by Galphaq. These findings demonstrate a scaffolding role for MLK3 in controlling the extent of Rho activation that modulates cell migration. Moreover, they suggest that MLK3 functions as a network hub that links a number of signaling pathways.

Mixed lineage kinase 3 connects reactive oxygen species to c-Jun NH2-terminal kinase-induced mitochondrial apoptosis in genipin-treated PC3 human prostate cancer cells

It has been reported that genipin, the aglycone of geniposide, induces apoptotic cell death in human hepatoma cells via a NADPH oxidase-reactive oxygen species (ROS)-c-Jun NH(2)-terminal kinase (JNK)-dependent activation of mitochondrial pathway. This continuing work aimed to define that mixed lineage kinase 3 (MLK3) is a key mediator, which connect between ROS and JNK in genipin-induced cell death signaling. In PC3 human prostate cancer cells, genipin stimulated MLK3 activity in concentration- and time-dependent manner. The PC3 cells stably transfected with dominant-negative form of MLK3 was less susceptible to population of the sub-G1 apoptotic cells, activation of caspase, collapse of mitochondrial membrane potential, and release of cytochrome c triggered by genipin, suggesting a crucial role of MLK3 in genipin signaling to apoptotic cell death. Diphenyleneiodonium (DPI), a specific inhibitor of NADPH oxidase, markedly inhibited ROS generation and MLK3 phosphorylation in the genipin-treated cells. Pretreatment with SP0600125, a specific inhibitor of JNK but neither U0126, a specific inhibitor of MEK1/2 nor PD169316, a specific inhibitor of p38 suppressed genipin-induced apoptotic cell death. Notably, both the phosphorylation of JNK and induction of c-Jun induced by genipin were markedly inhibited in PC3-EGFP-MLK3 (K144R) cells expressing a dominant-negative MLK3 mutant. Taken together, our observations suggest genipin signaling to apoptosis of PC3 cells is mediated via activation of ROS-dependent MLK3, which leads to downstream activation of JNK.