MLK3 localizes mainly to the cytoplasm and promotes oxidative stress injury via a positive feedback loop

Activation of mixed lineage kinase 3 (MLK3) by phosphorylation at Thr277/Ser281 stimulates downstream apoptotic pathways and ultimately leads to cell injury. MLK3 is reported to localize to both the cytoplasm and nucleus in human ovarian cancer cells and immortalized ovarian epithelial cells (T80 and T90 cells), and phosphorylation at Thr477 is required for the cytoplasmic retention of MLK3 in T80 cells. However, the subcellular distribution of MLK3 in other cell types has rarely been reported, and whether phosphorylation of MLK3 at Thr277/Ser281 affects its subcellular distribution is unknown. Here, our bioinformatics analysis predicted that MLK3 was mainly distributed in the cytoplasm and nucleus. In the human HEK293T embryonic kidney cell line and murine HT22 hippocampal neuronal cell line, endogenous MLK3 was more abundant in the cytoplasm and less abundant in the nucleus. In addition, overexpressed Myc-tagged MLK3 and EGFP-tagged MLK3 were also observed to localize mainly to the cytoplasm. MLK3 that was activated by phosphorylation at Thr277/Ser281 was mainly distributed in the cytoplasm, and phosphorylation deficient (T277A/S281A) and mimic (T277E/S281E) mutants both showed distributions similar to that of wild type (wt) MLK3, further proving that phosphorylation at Thr277/Ser281 was not involved in regulating MLK3 subcellular localization. In HEK293T cells, H2O2 stimulation accelerated MLK3 phosphorylation (activation), and this phosphorylation was reduced by the antioxidant N-acetylcysteine in a dose-dependent manner. Overexpressing wt MLK3 promoted the production of intracellular reactive oxygen species and increased cell apoptosis, both of which were enhanced by the phosphorylation-mimic (T277E/S281E) MLK3 variant but not by the phosphorylation-deficient (T277A/S281A) MLK3 variant. These findings provided additional evidence for the cytoplasmic and nuclear distribution of MLK3 in HEK293T cells or HT22 cells and revealed the pivotal role of MLK3 in the positive feedback loop of oxidative stress injury.

Stress-induced pseudokinase TRB3 augments IL1β signaling by interacting with Flightless homolog 1

Interleukin-1β is one of the most potent inducers of beta cell inflammation in the lead-up to type 1 diabetes. We have previously reported that IL1β-stimulated pancreatic islets from mice with genetic ablation of stress-induced pseudokinase TRB3(TRB3KO) show attenuated activation kinetics for the MAP3K MLK3 and JNK stress kinases. However, JNK signaling constitutes only a portion of the cytokine-induced inflammatory response. Here we report that TRB3KO islets also show a decrease in amplitude and duration of IL1β-induced phosphorylation of TAK1 and IKK, kinases that drive the potent NF-κB proinflammatory signaling pathway. We observed that TRB3KO islets display decreased cytokine-induced beta cell death, preceded by a decrease in select downstream NF-κB targets, including iNOS/NOS2 (inducible nitric oxide synthase), a mediator of beta cell dysfunction and death. Thus, loss of TRB3 attenuates both pathways required for a cytokine-inducible, proapoptotic response in beta cells. In order to better understand the molecular basis of TRB3-enhanced, post-receptor IL1β signaling, we interrogated the TRB3 interactome using coimmunoprecipitation followed by mass spectrometry to identify immunomodulatory protein Flightless homolog 1 (Fli1) as a novel, TRB3-interacting protein. We show that TRB3 binds and disrupts Fli1-dependent sequestration of MyD88, thereby increasing availability of this most proximal adaptor required for IL1β receptor-dependent signaling. Fli1 sequesters MyD88 in a multiprotein complex resulting in a brake on the assembly of downstream signaling complexes. By interacting with Fli1, we propose that TRB3 lifts the brake on IL1β signaling to augment the proinflammatory response in beta cells.

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.

PP1γ regulates neuronal insulin signaling and aggravates insulin resistance leading to AD-like phenotypes

BACKGROUND

PP1γ is one of the isoforms of catalytic subunit of a Ser/Thr phosphatase PP1. The role of PP1γ in cellular regulation is largely unknown. The present study investigated the role of PP1γ in regulating neuronal insulin signaling and insulin resistance in neuronal cells. PP1 was inhibited in mouse neuroblastoma cells (N2a) and human neuroblastoma cells (SH-SY5Y). The expression of PP1α and PP1γ was determined in insulin resistant N2a, SH-SY5Y cells and in high-fat-diet-fed-diabetic mice whole-brain-lysates. PP1α and PP1γ were silenced by siRNA in N2a and SH-SY5Y cells and effect was tested on AKT isoforms, AS160 and GSK3 isoforms using western immunoblot, GLUT4 translocation by confocal microscopy and glucose uptake by fluorescence-based assay.

RESULTS

Results showed that, in one hand PP1γ, and not PP1α, regulates neuronal insulin signaling and insulin resistance by regulating phosphorylation of AKT2 via AKT2-AS160-GLUT4 axis. On the other hand, PP1γ regulates phosphorylation of GSK3β via AKT2 while phosphorylation of GSK3α via MLK3. Imbalance in this regulation results into AD-like phenotype.

CONCLUSION

PP1γ acts as a linker, regulating two pathophysiological conditions, neuronal insulin resistance and AD. Video Abstract.

Mixed lineage kinase 3 and CD70 cooperation sensitize trastuzumab-resistant HER2(+) breast cancer by ceramide-loaded nanoparticles

Trastuzumab is the first-line therapy for human epidermal growth factor receptor 2-positive (HER2⁺) breast cancer, but often patients develop acquired resistance. Although other agents are in clinical use to treat trastuzumab-resistant (TR) breast cancer; still, the patients develop recurrent metastatic disease. One of the primary mechanisms of acquired resistance is the shedding/loss of the HER2 extracellular domain, where trastuzumab binds. We envisioned any new agent acting downstream of the HER2 should overcome trastuzumab resistance. The mixed lineage kinase 3 (MLK3) activation by trastuzumab is necessary for promoting cell death in HER2⁺ breast cancer. We designed nanoparticles loaded with MLK3 agonist ceramide (PPP-CNP) and tested their efficacy in sensitizing TR cell lines, patient-derived organoids, and patient-derived xenograft (PDX). The PPP-CNP activated MLK3, its downstream JNK kinase activity, and down-regulated AKT pathway signaling in TR cell lines and PDX. The activation of MLK3 and down-regulation of AKT signaling by PPP-CNP induced cell death and inhibited cellular proliferation in TR cells and PDX. The apoptosis in TR cells was dependent on increased CD70 protein expression and caspase-9 and caspase-3 activities by PPP-CNP. The PPP-CNP treatment alike increased the expression of CD70, CD27, cleaved caspase-9, and caspase-3 with a concurrent tumor burden reduction of TR PDX. Moreover, the expressions of CD70 and ceramide levels were lower in TR than sensitive HER2⁺ human breast tumors. Our in vitro and preclinical animal models suggest that activating the MLK3-CD70 axis by the PPP-CNP could sensitize/overcome trastuzumab resistance in HER2⁺ breast cancer.

MLK3 Regulates Inflammatory Response via Activation of AP-1 Pathway in HEK293 and RAW264.7 Cells

Inflammation is a critically important barrier found in innate immunity. However, severe and sustained inflammatory conditions are regarded as causes of many different serious diseases, such as cancer, atherosclerosis, and diabetes. Although numerous studies have addressed how inflammatory responses proceed and what kinds of proteins and cells are involved, the exact mechanism and protein components regulating inflammatory reactions are not fully understood. In this paper, to determine the regulatory role of mixed lineage kinase 3 (MLK3), which functions as mitogen-activated protein kinase kinase kinase (MAP3K) in cancer cells in inflammatory response to macrophages, we employed an overexpression strategy with MLK3 in HEK293 cells and used its inhibitor URMC-099 in lipopolysaccharide (LPS)-treated RAW264.7 cells. It was found that overexpressed MLK3 increased the mRNA expression of inflammatory genes (COX-2, IL-6, and TNF-α) via the activation of AP-1, according to a luciferase assay carried out with AP-1-Luc. Overexpression of MLK3 also induced phosphorylation of MAPKK (MEK1/2, MKK3/6, and MKK4/7), MAPK (ERK, p38, and JNK), and AP-1 subunits (c-Jun, c-Fos, and FRA-1). Phosphorylation of MLK3 was also observed in RAW264.7 cells stimulated by LPS, Pam3CSK, and poly(I:C). Finally, inhibition of MLK3 by URMC-099 reduced the expression of COX-2 and CCL-12, phosphorylation of c-Jun, luciferase activity mediated by AP-1, and phosphorylation of MAPK in LPS-treated RAW264.7 cells. Taken together, our findings strongly suggest that MLK3 plays a central role in controlling AP-1-mediated inflammatory responses in macrophages and that this enzyme can serve as a target molecule for treating AP-1-mediated inflammatory diseases.

Mixed Lineage Kinase 3 phosphorylates prolyl-isomerase PIN1 and potentiates GLI1 signaling in pancreatic cancer development

The transcription factor Glioma-Associated Oncogene Homolog 1 (GLI1) is activated by sonic hedgehog (SHH) cascade and is an established driver of pancreatic ductal adenocarcinoma (PDAC). However, therapies targeting upstream hedgehog signaling have shown little to no efficacy in clinical trials. Here, we identify Mixed Lineage Kinase 3 (MLK3) as a druggable regulator of oncogenic GLI1. Earlier, we reported that MLK3 phosphorylated a peptidyl-prolyl isomerase PIN1 on the S138 site, and the PIN1-pS138 translocated to the nucleus. In this report, we identify GLI1 as one of the targets of PIN1-pS138 and demonstrate that PIN1-pS138 is upregulated in human PDAC and strongly associates with the upregulation of GLI1 and MLK3 expression. Moreover, we also identified two new phosphorylation sites on GLI1, T394, and S1089, which are directly phosphorylated by MLK3 to promote GLI1 nuclear translocation, transcriptional activity, and cell proliferation. Additionally, pharmacological inhibition of MLK3 by CEP-1347 promoted apoptosis in PDAC cell lines, reduced tumor burden, extended survival, and reduced GLI1 expression in the Pdx1-Cre x LSL-KRASG12D x LSL-TP53R172H (KPC) mouse model of PDAC. These findings collectively suggest that MLK3 is an important regulator of oncogenic GLI1 and that therapies targeting MLK3 warrant consideration in the management of PDAC patients.

Neuroprotection of sevoflurane against ischemia/reperfusion-induced brain injury through inhibiting GluN2A/GluN2B-PSD-95-MLK3 module

To investigate the role of GluN2A and GluN2B in neuroprotective effect of sevoflurane preconditioning against cerebral ischemia-reperfusion injury (CIRI). Rats were randomly divided into five groups as follows: control, ischemia-reperfusion (I/R) 6 h, sevoflurane preconditioning (SP), SP + amantadine, SP + NMDA. Immunoblot and immunoprecipitation were used to detect the tyrosine phosphorylation of GluN2A/GluN2B, the interaction of GluN2A/GluN2B-PSD-95-MLK3 and the expression of phosphorylation of MLK3, MKK7 and JNK3. Cresyl violet staining was employed to analyse neuronal injury in rat hippocampal CA1 subfields. Sevoflurane preconditioning inhibits the tyrosine phosphorylation of GluN2A/GluN2B, the interaction of GluN2A/GluN2B-PSD-95-MLK3 and the phosphorylation of MLK3, MKK7 and JNK3 in rat hippocampus. An N-methyl-D-aspartate receptor (NMDAR) antagonist amantadine reversed the MLK3-MKK7- JNK3 signal events. Such reversion was also realized by NMDA (60 and 80 nmol) and low doses of NMDA (0-40 nmol) could not change the inhibitory effect of sevoflurane preconditioning on MLK3-MKK7-JNK3 signal events. Finally, Cresyl violet staining also confirmed that low dose of NMDA reduced neuronal loss in rat hippocampal CA1 subfields. Sevoflurane preconditioning provides neuroprotection against CIRI by inhibiting NMDAR over-activation.

Xinyang Tablet inhibits MLK3-mediated pyroptosis to attenuate inflammation and cardiac dysfunction in pressure overload

ETHNOPHARMACOLOGICAL RELEVANCE

Xinyang tablet (XYT) has been traditionally used in the treatment of cardiovascular diseases (CVDs). Our previous study indicated that XYT exhibited protective effects in heart failure (HF).

AIM OF THE STUDY

The aim of the present study was to determine the protective effects of XYT in pressure overload induced HF and to elucidate its underlying mechanisms of action.

MATERIALS AND METHODS

We analyzed XYT content using high-performance liquid chromatography (HPLC.). Mice were subjected to transverse aortic constriction (TAC) to generate pressure overload-induced cardiac remodeling and were then orally administered XYT or URMC-099 for 1 week after the operation. HL1 mouse cardiomyoblasts were induced by lipopolysaccharides (LPS) to trigger pyroptosis and were then treated with XYT or URMC-099. We used echocardiography (ECG), hematoxylin and eosin (H&E) staining, Masson's trichrome staining and a terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) assay to evaluate the effects of XYT. Messenger ribonucleic acid (mRNA) levels of collagen metabolism biomarkers and inflammation-related factors were detected. We determined protein levels of inflammation- and pyroptosis-related signaling pathway members via Western blot (WB). Caspase-1 activity was measured in cell lysate using a Caspase-1 Activity Assay Kit. Subsequently, to define the candidate ingredients in XYT that regulate mixed-lineage kinase-3 (MLK3), we used molecular docking (MD) to predict and evaluate binding affinity with MLK3. Finally, we screened 24 active potential compounds that regulate MLK3 via MD.

RESULTS

ECG, H&E staining, Masson's trichrome staining and TUNEL assay results showed that XYT remarkably improved heart function, amelorated myocardial fibrosis and inhibited apoptosis in vivo. Moreover, it reduced expression of proteins or mRNAs related to collagen metabolism, including collagen type 1 (COL1), fibronectin (FN), alpha smooth-muscle actin (α-SMA), and matrix metalloproteinases-2 and -9 (MMP-2, MMP-9). XYT also inhibited inflammation and the induction of pyroptosis at an early stage, as well as attenuated inflammation and pyroptosis levels in vitro.

CONCLUSION

Our data indicated that XYT exerted protective effects against pressure overload induced myocardial fibrosis (MF), which might be associated with the induction of pyroptosis-mediated MLK3 signaling.

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.

Synergism between the phosphatidylinositol 3-kinase p110β isoform inhibitor AZD6482 and the mixed lineage kinase 3 inhibitor URMC-099 on the blockade of glioblastoma cell motility and focal adhesion formation

Background

Glioblastoma multiforme, the most aggressive and malignant primary brain tumor, is characterized by rapid growth and extensive infiltration to neighboring normal brain parenchyma. Our previous studies delineated a crosstalk between PI3K/Akt and JNK signaling pathways, and a moderate anti-glioblastoma synergism caused by the combined inhibition of PI3K p110β (PI3Kβ) isoform and JNK. However, this combination strategy is not potent enough. MLK3, an upstream regulator of ERK and JNK, may replace JNK to exert stronger synergism with PI3Kβ.

Methods

To develop a new combination strategy with stronger synergism, the expression pattern and roles of MLK3 in glioblastoma patient’s specimens and cell lines were firstly investigated. Then glioblastoma cells and xenografts in nude mice were treated with the PI3Kβ inhibitor AZD6482 and the MLK3 inhibitor URMC-099 alone or in combination to evaluate their combination effects on tumor cell growth and motility. The combination effects on cytoskeletal structures such as lamellipodia and focal adhesions were also evaluated.

Results

MLK3 protein was overexpressed in both newly diagnosed and relapsing glioblastoma patients’ specimens. Silencing of MLK3 using siRNA duplexes significantly suppressed migration and invasion, but promoted attachment of glioblastoma cells. Combined inhibition of PI3Kβ and MLK3 exhibited synergistic inhibitory effects on glioblastoma cell proliferation, migration and invasion, as well as the formation of lamellipodia and focal adhesions. Furthermore, combination of AZD6482 and URMC-099 effectively decreased glioblastoma xenograft growth in nude mice. Glioblastoma cells treated with this drug combination showed reduced phosphorylation of Akt and ERK, and decreased protein expression of ROCK2 and Zyxin.

Conclusion

Taken together, combination of AZD6482 and URMC-099 showed strong synergistic anti-tumor effects on glioblastoma in vitro and in vivo. Our findings suggest that combined inhibition of PI3Kβ and MLK3 may serve as an attractive therapeutic approach for glioblastoma multiforme.

Mixed lineage kinase 3 inhibition induces T cell activation and cytotoxicity

Mixed lineage kinase 3 (MLK3), also known as MAP3K11, was initially identified in a megakaryocytic cell line and is an emerging therapeutic target in cancer, yet its role in immune cells is not known. Here, we report that loss or pharmacological inhibition of MLK3 promotes activation and cytotoxicity of T cells. MLK3 is abundantly expressed in T cells, and its loss alters serum chemokines, cytokines, and CD28 protein expression on T cells and its subsets. MLK3 loss or pharmacological inhibition induces activation of T cells in in vitro, ex vivo, and in vivo conditions, irrespective of T cell activating agents. Conversely, overexpression of MLK3 decreases T cell activation. Mechanistically, loss or inhibition of MLK3 down-regulates expression of a prolyl-isomerase, Ppia, which is directly phosphorylated by MLK3 to increase its isomerase activity. Moreover, MLK3 also phosphorylates nuclear factor of activated T cells 1 (NFATc1) and regulates its nuclear translocation via interaction with Ppia, and this regulates T cell effector function. In an immune-competent mouse model of breast cancer, MLK3 inhibitor increases Granzyme B-positive CD8+ T cells and decreases MLK3 and Ppia gene expression in tumor-infiltrating T cells. Likewise, the MLK3 inhibitor in pan T cells, isolated from breast cancer patients, also increases cytotoxic CD8+ T cells. These results collectively demonstrate that MLK3 plays an important role in T cell biology, and targeting MLK3 could serve as a potential therapeutic intervention via increasing T cell cytotoxicity in cancer.

Arabidopsis thaliana MLK3, a Plant-specific Casein Kinase 1, Negatively Regulates Flowering and Phosphorylates Histone H3 in Vitro

Arabidopsis thalianaMUT9-LIKE KINASES (MLKs), a family of the plant-specific casein kinase 1 (CK1), have been implicated collectively in multiple biological processes including flowering. Three of the four MLKs (MLK1/2/4) have been characterized, however, little is known about MLK3, the most divergent member of MLKs. Here, we demonstrated that disruption of MLK3 transcript in mlk3 caused early flowering with retarded leaf growth under long-day conditions. In vitro kinase assay showed the nuclear protein MLK3 phosphorylated histone 3 at threonine 3 (H3T3) and mutation of a conserved residue (K146R) abolished the catalytic activity. Ectopic expression of MLK3 but not MLK3(K146R) rescued the morphological defects of mlk3, indicating that an intact MLK3 is critical for maintaining proper flowering time. Transcriptomic analysis revealed that the floral repressor FLOWERING LOCUS C (FLC) was down-regulated significantly in mlk3, suggesting that MLK3 negatively regulates flowering. Hence, MLK3 plays a role in repressing the transition from vegetative to reproductive phase in A. thaliana. This study sheds light on the delicate control of flowering time by A. thaliana CK1 specific to the plant kingdom.

Anthocyanins attenuate neuroinflammation through the suppression of MLK3 activation in a mouse model of perioperative neurocognitive disorders

Perioperative neurocognitive disorder (PND) is a common complication that lacks effective prevention and treatment measures. Neuroinflammation is considered to be one of the important mechanisms of PND. In this study, we investigated the effect and mechanism of anthocyanins (ANT), a natural plant ingredient, on postoperative cognition and neuroinflammation. Mice were subjected to laparotomy and treated with ANT (50-100 mg/kg, orally) 4 weeks before surgery and then once per day for 7 days after surgery. Morris water maze and fear conditioning tests were used to evaluate learning and memory function. Western blotting, RT-PCR and immunofluorescence were used to test the neuroprotective effects and mechanism of ANT. The results showed that ANT improved learning and memory in mice after surgery. ANT also inhibited MLK3 activation and its downstream JNK and p38 MAPK signaling cascades. Moreover, the inhibitor of MLK3 could mimic the effects of ANT. In addition, treatment with ANT significantly reduced neuroinflammation and microglia activation. In conclusion, MLK3 represents a novel target in surgery-induced neuroinflammation and cognitive dysfunction. ANT could inhibit the activation of MLK3 and be a promising agent for the prevention and treatment of PND.

High-Throughput Screening Identifies Mixed-Lineage Kinase 3 as a Key Host Regulatory Factor in Zika Virus Infection

The Zika virus (ZIKV) life cycle involves multiple steps and requires interactions with host factors. However, the inability to systematically identify host regulatory factors for ZIKV has hampered antiviral development and our understanding of pathogenicity. Here, using a bioactive compound library with 2,659 small molecules, we applied a high-throughput and imaging-based screen to identify host factors that modulate ZIKV infection. The screen yielded hundreds of hits that markedly inhibited or potentiated ZIKV infection in SNB-19 glioblastoma cells. Among the hits, URMC-099, a mixed-lineage kinase 3 (MLK3) inhibitor, significantly facilitated ZIKV replication in both SNB-19 cells and the neonatal mouse brain. Using gene silencing and overexpression, we further confirmed that MLK3 was a host restriction factor against ZIKV. Mechanistically, MLK3 negatively regulated ZIKV replication through induction of the inflammatory cytokines interleukin-6 (IL-6), IL-8, tumor necrosis factor alpha (TNF-α), and monocyte chemoattractant protein 1 (MCP-1) but did not modulate host interferon-related pathways. Importantly, ZIKV activated the MLK3/MKK7/Jun N-terminal protein kinase (JNK) pathway in both SNB-19 cells and neonatal mouse brain. Together, these findings reveal a critical role for MLK3 in regulating ZIKV infection and facilitate the development of anti-ZIKV therapeutics by providing a number of screening hits.IMPORTANCE Zika fever, an infectious disease caused by the Zika virus (ZIKV), normally results in mild symptoms. Severe infection can cause Guillain-Barré syndrome in adults and birth defects, including microcephaly, in newborns. Although ZIKV was first identified in Uganda in 1947 in rhesus monkeys, a widespread epidemic of ZIKV infection in South and Central America in 2015 and 2016 raised major concerns. To date, there is no vaccine or specific medicine for ZIKV. The significance of our research is the systematic discovery of small molecule candidates that modulate ZIKV infection, which will allow the development of antiviral therapeutics. In addition, we identified MLK3, a key mediator of host signaling pathways that can be activated during ZIKV infection and limits virus replication by inducing multiple inflammatory cytokines. These findings broaden our understanding of ZIKV pathogenesis.

Suppression of the RAC1/MLK3/p38 Signaling Pathway by β-Elemene Alleviates Sepsis-Associated Encephalopathy in Mice

It is still difficult to treat sepsis-associated encephalopathy (SAE) which is a diffuse brain dysfunction caused by sepsis, with excessive activation of microglia as one of the main mechanisms. Ras-related C3 botulinum toxin substrate 1 (RAC1) is proven to be a key molecule in the inflammatory signaling network. By using microglial cell line BV-2 and a mouse model of cecal ligation puncture (CLP), we herein evaluated the effects of β-elemene, an extract of Curcuma zedoaria Rosc., on RAC1 signaling in microglia. β-Elemene decreased the expressions of pro-inflammatory cytokines [tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and IL-6] and attenuated translocation of nuclear factor-κB (NF-κB) p65 from the cytosol to the nucleus in BV-2 cells after lipopolysaccharide (LPS) treatment. It also inhibited the activation of RAC1, mixed-lineage protein kinase 3 (MLK3) and p38 mitogen-activated protein kinase (MAPK). The phosphorylation of the RAC1 Ser71 site was increased by β-elemene. Moreover, the learning and memory abilities of CLP mice in the water maze test and fear conditioning test were improved after β-elemene treatment. It reduced the expression of the microglial marker IBA1, significantly increased RAC1 Ser71 phosphorylation, and suppressed the RAC1/MLK3/p38 signaling activation and inflammatory response in the hippocampus. In conclusion, β-elemene effectively alleviated SAE in mice and inhibited the RAC1/MLK3/p38 signaling pathway in microglia, and might be an eligible potential candidate for SAE treatment.

miR-140-5p aggravates hypoxia-induced cell injury via regulating MLK3 in H9c2 cells

Myocardial infarction (MI) is an important cause of cardiovascular disease. microRNAs (miRNAs) have been indicated as pivotal regulators in the physiological and pathological processes of heart diseases. The purpose of this study was to investigate the role of miR-140-5p in hypoxia-induced cell injury in H9c2 cells and its underlying mechanism. H9c2 cells were subjected to hypoxia, before which the expression levels of miR-140-5p and MLK3 were overexpressed or knocked down through transient transfection. The efficiency of transfection was verified by qRT-PCR and Western blotting. Cell viability, apoptotic cell rate, and the expression changes of apoptosis-related proteins were determined by trypan blue exclusion, flow cytometry, and Western blotting, respectively. Furthermore, Western blotting was performed to assess the expression levels of core factors related with p38MAPK and JNK signaling pathways. As a result, hypoxia significantly reduced cell viability and increased cell apoptosis in H9c2 cells. miR-140-5p inhibition attenuated cell injury induced by hypoxia in H9c2 cells, while miR-140-5p overexpression expedited the cell injury, as evidenced by the decreased cell viability and enhanced cell apoptosis. Moreover, miR-140-5p promoted the activation of p38MAPK and JNK pathways. miR-140-5p positively modulated the expression of MLK3. ML3 overexpression reversed the regulatory effects of miR-140-5p inhibition on hypoxia-injured H9c2 cells. In conclusion, this study demonstrated that miR-140-5p aggravated hypoxia-induced cell injury partially through up-regulation of MLK3.

Mixed Lineage Kinase 3 Mediates the Induction of CXCL10 by a STAT1-Dependent Mechanism During Hepatocyte Lipotoxicity

Saturated fatty acids (SFA) and their toxic metabolites contribute to hepatocyte lipotoxicity in nonalcoholic steatohepatitis (NASH). We previously reported that hepatocytes, under lipotoxic stress, express the potent macrophage chemotactic ligand C-X-C motif chemokine 10 (CXCL10), and release CXCL10-enriched extracellular vesicles (EV) by a mixed lineage kinase (MLK) 3-dependent mechanism. In the current study, we sought to examine the signaling pathway responsible for CXCL10 induction during hepatocyte lipotoxicity. Here, we demonstrate a role for signal transducer and activator of transcription (STAT) 1 in regulating CXCL10 expression. Huh7 and HepG2 cells were treated with lysophosphatidylcholine (LPC), the toxic metabolite of the SFA palmitate. In LPC-treated hepatocytes, CXCL10 induction is mediated by a mitogen activated protein kinase (MAPK) signaling cascade consisting of a relay kinase module of MLK3, MKK3/6, and p38. P38 in turn induces STAT1 Ser727 phosphorylation and CXCL10 upregulation in hepatocytes, which is reduced by genetic or pharmacological inhibition of this MAPK signaling cascade. The binding and activity of STAT1 at the CXCL10 gene promoter were identified by chromatin immunoprecipitation and luciferase gene expression assays. Promoter activation was attenuated by MLK3/STAT1 inhibition or by deletion of the consensus STAT1 binding sites within the CXCL10 promoter. In lipotoxic hepatocytes, MLK3 activates a MAPK signaling cascade, resulting in the activating phosphorylation of STAT1, and CXCL10 transcriptional upregulation. Hence, this kinase relay module and/or STAT1 inhibition may serve as a therapeutic target to reduce CXCL10 release, thereby attenuating NASH pathogenesis. J. Cell. Biochem. 118: 3249-3259, 2017. © 2017 Wiley Periodicals, Inc.

Transcriptional regulation of mixed lineage kinase 3 by estrogen and its implication in ER-positive breast cancer pathogenesis

Mixed Lineage Kinase 3 (MLK3), also called as MAP3K11 is a tightly regulated MAP3K member but its cellular function is still not fully understood. Earlier we reported post-translational regulation of MLK3 by estrogen (E2) that inhibited the kinase activity and favored survival of ER+ breast cancer cells. Here we report that MLK3 is also transcriptionally downregulated by E2 in ER+ breast cancer cells. Publicly available data and in situ hybridization of human breast tumors showed significant down regulation of MLK3 transcripts in ER+ tumors. The basal level of MLK3 transcripts and protein in ER+ breast cancer cell lines were significantly lower, and the protein expression was further down regulated by E2 in a time-dependent manner. Analysis of the promoter of MLK3 revealed two ERE sites which were regulated by E2 in ER+ but not in ER- breast cancer cell lines. Both ERα and ERβ were able to bind to MLK3 promoter and recruit nuclear receptor co-repressors (NCoR, SMRT and LCoR), leading to down-regulation of MLK3 transcripts. Collectively these results suggest that recruitment of nuclear receptor co-repressor is a key feature of ligand-dependent transcriptional repression of MLK3 by ERs. Therefore coordinated transcriptional and post-translational repression of pro-apoptotic MLK3 probably is one of the mechanisms by which ER+ breast cancer cells proliferate and survive.

IDH1 Mutation Promotes Tumorigenesis by Inhibiting JNK Activation and Apoptosis Induced by Serum Starvation

Two hallmarks of cancer cells are their resistance to apoptosis and ability to thrive despite reduced levels of vital serum components. c-jun N-terminal kinase (JNK) activation is crucial for apoptosis triggered by serum starvation (SS), and isocitrate dehydrogenase 1 (IDH1) mutations are tumorigenic, in part, because they produce the abnormal metabolite 2-hydroxyglutarate (2-HG). However, it is unknown whether 2-HG-induced tumorigenesis is partially due to JNK inhibition and thus defective SS-induced apoptosis. We show here, using IDH1-R132Q knockin mutant mouse cells, that 2-HG inhibits JNK activation induced only by SS and not by UV or doxorubicin, and thus can block apoptosis. Upon SS, Cdc42 normally disrupts mixed lineage kinase 3's (MLK3's) auto-inhibition, triggering the MLK3-MKK4/7-JNK-Bim apoptotic cascade. 2-HG binds to Cdc42 and abolishes its association with MLK3, inactivating MLK3 and apoptosis. Allograft tumor assays in mice demonstrate that this mechanism contributes to tumorigenesis driven by mutant IDH1, a result confirmed by detection of JNK inactivation in human gliomas harboring IDH1-R132H mutations.

Long non-coding RNA small nucleolar RNA host gene 12 (SNHG12) promotes tumorigenesis and metastasis by targeting miR-199a/b-5p in hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is third leading cause of cancer-related death globally. Evidence suggest that long non-coding RNAs (lncRNAs) have emerged as key regulators of tumorigenesis and metastasis in HCC. In this study, we investigated the functional significance of SNHG12 and explored whether SNHG12 can directly interact with miR-199a/b-5p in the progression of HCC.

Methods

We determined the expression level of SNHG12 in HCC tissues with quantitative real-time PCR and then studied its clinical significance. The binding site between SNHG12 and miR-199a/b-5p was confirmed using dual luciferase assay and RNA immunoprecipitation (RIP) assay. SNHG12 was silenced through the siRNA transfection to determine whether SNHG12-siRNA is able to affect cell proliferation, invasion and metastasis.

Results

SNHG12 was significantly higher in the HCC tissues than that in the adjacent normal tissues. There were direct interactions between miR-199a/b-5p and the binding site of SNHG12. SNHG12 functioned as an endogenous sponge for miR-199a/b-5p to regulate the expression of MLK3 and affect the NF-κB pathway.

Conclusion

SNHG12 may serve as a valuable biomarker and a potential therapeutic target for HCC.

MAP3K11/GDF15 axis is a critical driver of cancer cachexia

BACKGROUND

Cancer associated cachexia affects the majority of cancer patients during the course of the disease and thought to be directly responsible for about a quarter of all cancer deaths. Current evidence suggests that a pro-inflammatory state may be associated with this syndrome although the molecular mechanisms responsible for the development of cachexia are poorly understood. The purpose of this work was the identification of key drivers of cancer cachexia that could provide a potential point of intervention for the treatment and/or prevention of this syndrome.

METHODS

Genetically engineered and xenograft tumour models were used to dissect the molecular mechanisms driving cancer cachexia. Cytokine profiling from the plasma of cachectic and non-cachectic cancer patients and mouse models was utilized to correlate circulating cytokine levels with the cachexia phenotype.

RESULTS

Utilizing engineered tumour models we identified MAP3K11/GDF15 pathway activation as a potent inducer of cancer cachexia. Increased expression and high circulating levels of GDF15 acted as a key mediator of this process. In animal models, tumour-produced GDF15 was sufficient to trigger the cachexia phenotype. Elevated GDF15 circulating levels correlated with the onset and progression of cachexia in animal models and in patients with cancer. Inhibition of GDF15 biological activity with a specific antibody reversed body weight loss and restored muscle and fat tissue mass in several cachectic animal models regardless of their complex secreted cytokine profile.

CONCLUSIONS

The combination of correlative observations, gain of function, and loss of function experiments validated GDF15 as a key driver of cancer cachexia and as a potential therapeutic target for the treatment and/or prevention of this syndrome.

Protective role of downregulated MLK3 in myocardial adaptation to chronic hypoxia

A series of protective responses could be evoked to achieve compensatory adaptation once cardiomyocytes are subjected to chronic hypoxia. MLK3/JNK/c-jun signaling pathway was previously demonstrated to be involved in this process. In the present study, we aim to further examine the performance of MLK3 in hypoxic H9C2 cells and potential mechanism. Myocardial samples of patients with congenital heart disease (CHD) were collected. H9C2 cells were cultured in hypoxic conditions for various durations. MLK3 was silenced by transfection of shRNA to evaluate its role in cell viability. We found expression of MLK3 protein was lower in patients with cyanotic CHD. In hypoxic H9C2 cells, its expression was gradually decreased in a time-dependent manner. However, there was no significant difference about expression of MLK3 mRNA. According to the results of MTT, LDH, and TUNEL, faster cell growth curve, lower death rate, and less apoptotic cells could be observed in MLK-shRNA group compared with scramble-shRNA group. Silencing of MLK3 significantly reduced expression of cleaved caspase-3, cleaved PARP, Bad, and Bax, together with increased expression of Bcl-2 and ration of Bcl-2/Bax. Both ratio of phospho-JNK/total JNK and ratio of phospho-c-jun/total c-jun were significantly decreased once MLK3 was silenced. At various reoxygenation time, MLK3 shRNA could significantly promote cell survival and decrease cell death according to MTT and LDH. Our results suggested that chronic hypoxia could reduce MLK3 expression in a posttranscriptional regulatory manner. Downregulation of MLK3 protects H9C2 cells from hypoxia-induced apoptosis and H/R injury via blocking the activation of JNK and c-jun.

Decrement of miR-199a-5p contributes to the tumorigenesis of bladder urothelial carcinoma by regulating MLK3/NF-κB pathway

Aberrant miRNA expression is implicated in tumorigenesis. However, the role of miRNAs in bladder urothelial carcinoma still remains largely unknown. In this study, miR-199a-5p was validated to be significantly down-regulated in bladder urothelial carcinoma. In addition, restoring expression of miR-199a-5p inhibited the tumorigenesis of bladder urothelial carcinoma in vitro and in vivo by inducing the apoptosis and suppressing the proliferation of bladder cancerous cells. Further investigation reported that MLK3 was a direct target of miR-199a-5p. Moreover, the expression level of miR-199a-5p was conversely correlated with MLK3 in bladder cancerous cells. In addition, reintroduction of MLK3 was identified to promote the proliferation and inhibit the apoptotic rate of cells, which have been altered by miR-199a-5p through activating the NF-κB pathway. All together, decrement of miR-199a-5p contributes to the tumorigenesis of bladder cancer by directly regulating MLK3/NF-κB pathway and miR-199a-5p might be developed as a therapeutic target for treatment of the bladder urothelial carcinoma.

Overexpression of C-terminal fragment of glutamate receptor 6 prevents neuronal injury in kainate-induced seizure via disassembly of GluR6-PSD-95-MLK3 signaling module

Our previous study showed that when glutamate receptor (GluR)6 C terminus-containing peptide conjugated with the human immunodeficiency virus Tat protein (GluR6)-9c is delivered into hippocampal neurons in a brain ischemic model, the activation of mixed lineage kinase 3 (MLK3) and c-Jun NH2-terminal kinase (JNK) is inhibited via GluR6-postsynaptic density protein 95 (PSD95). In the present study, we investigated whether the recombinant adenovirus (Ad) carrying GluR6c could suppress the assembly of the GluR6-PSD95-MLK3 signaling module and decrease neuronal cell death induced by kainate in hippocampal CA1 subregion. A seizure model in Sprague-Dawley rats was induced by intraperitoneal injections of kainate. The effect of Ad-Glur6-9c on the phosphorylation of JNK, MLK3 and mitogen-activated kinase kinase 7 (MKK7) was observed with western immunoblots and immunohistochemistry. Our findings revealed that overexpression of GluR6c inhibited the interaction of GluR6 with PSD95 and prevented the kainate-induced activation of JNK, MLK3 and MKK7. Furthermore, kainate-mediated neuronal cell death was significantly suppressed by GluR6c. Taken together, GluR6 may play a pivotal role in neuronal cell death.

MLK3 regulates fMLP-stimulated neutrophil motility

INTRODUCTION

Mixed lineage kinase 3 (MLK3) is part of the intracellular regulatory system that connects extracellular cytokine or mitogen signals received through G-protein coupled receptors to changes in gene expression. MLK3 activation stimulates motility of epithelial cells and epithelial-derived tumor cells, but its role in mediating the migration of other cell types remains unknown. Since neutrophils play a crucial role in innate immunity and contribute to the pathogenesis of several diseases, we therefore examined whether MLK3 might regulate the motility of mouse neutrophils responding to a chemotactic stimulus, the model bacterial chemoattractant fMLP.

METHODS

The expression of Mlk3 in mouse neutrophils was determined by immunocytochemistry and by RT-PCR. In vitro chemotaxis in a gradient of fMLP, fMLP-stimulated random motility, fMLP-stimulated F-actin formation were measured by direct microscopic observation using neutrophils pre-treated with a novel small molecule inhibitor of MLK3 (URMC099) or neutrophils obtained from Mlk3-/- mice. In vivo effects of MLK3 inhibition were measured by counting the fMLP-induced accumulation of neutrophils in the peritoneum following pre-treatment with URMC099 in wild-type C57Bl/6 or mutant Mlk3-/- mice.

RESULTS

The expression of Mlk3 mRNA and protein was observed in neutrophils purified from wild-type C57Bl/6 mice but not in neutrophils from mutant Mlk3-/- mice. Chemotaxis by wild-type neutrophils induced by a gradient of fMLP was reduced by pre-treatment with URMC099. Neutrophils from C57Bl/6 mice pretreated with URMC099 and neutrophils from Mlk3-/- mice moved far less upon fMLP-stimulation and did not form F-actin as readily as untreated neutrophils from C57Bl/6 controls. In vivo recruitment of neutrophils into the peritoneum by fMLP was significantly reduced in wild-type mice treated with URMC099, as well as in untreated Mlk3-/- mice-thereby confirming the role of MLK3 in neutrophil migration.

CONCLUSIONS

Mlk3 mRNA is expressed in murine neutrophils. Genetic or pharmacologic inhibition of MLK3 blocks fMLP-mediated motility of neutrophils both in vitro and in vivo, suggesting that MLK3 may be a therapeutic target in human diseases characterized by exuberant neutrophil migration.

Connective tissue growth factor induces collagen I expression in human lung fibroblasts through the Rac1/MLK3/JNK/AP-1 pathway

Connective tissue growth factor (CTGF) plays an important role in lung fibrosis. In this study, we investigated the role of Rac1, mixed-lineage kinase 3 (MLK3), c-Jun N-terminal kinase (JNK), and activator protein-1 (AP-1) in CTGF-induced collagen I expression in human lung fibroblasts. CTGF caused concentration- and time-dependent increases in collagen I expression. CTGF-induced collagen I expression was inhibited by the dominant negative mutant (DN) of Rac1 (RacN17), MLK3DN, MLK3 inhibitor (K252a), JNK1DN, JNK2DN, a JNK inhibitor (SP600125), and an AP-1 inhibitor (curcumin). Treatment of cells with CTGF caused activation of Rac1, MLK3, JNK, and AP-1. The CTGF-induced increase in MLK3 phosphorylation was inhibited by RacN17. Treatment with RacN17 and the MLK3DN inhibited CTGF-induced JNK phosphorylation. CTGF caused increases in c-Jun phosphorylation and the recruitment of c-Jun and c-Fos to the collagen I promoter. Furthermore, stimulation of cells with the CTGF resulted in increases in AP-1-luciferase activity; this effect was inhibited by Rac1N17, MLK3DN, JNK1DN, and JNK2DN. Moreover, CTGF-induced α-smooth muscle actin (α-SMA) expression was inhibited by the procollagen I small interfering RNA (siRNA). These results suggest for the first time that CTGF acting through Rac1 activates the MLK3/JNK signaling pathway, which in turn initiates AP-1 activation and recruitment of c-Jun and c-Fos to the collagen I promoter and ultimately induces collagen I expression in human lung fibroblasts.

miR-138 protects cardiomyocytes from hypoxia-induced apoptosis via MLK3/JNK/c-jun pathway

Cardiomyocytes experience a series of complex endogenous regulatory mechanisms against apoptosis induced by chronic hypoxia. MicroRNAs are a class of endogenous small non-coding RNAs that regulate cellular pathophysiological processes. Recently, microRNA-138 (miR-138) has been found related to hypoxia, and beneficial for cell proliferation. Therefore, we intend to study the role of miR-138 in hypoxic cardiomyocytes and the main mechanism. Myocardial samples of patients with congenital heart disease (CHD) were collected to test miR-138 expression. Agomir or antagomir of miR-138 was transfected into H9C2 cells to investigate its effect on cell apoptosis. Higher miR-138 expression was observed in patients with cyanotic CHD, and its expression gradually increased with prolonged hypoxia time in H9C2 cells. Using MTT and LDH assays, cell growth was significantly greater in the agomir group than in the negative control (NC) group, while antagomir decreased cell survival. Dual luciferase reporter gene and Western-blot results confirmed MLK3 was a direct target of miR-138. It was found that miR-138 attenuated hypoxia-induced apoptosis using TUNEL, Hoechst staining and Annexin V-PE/7-AAD flow cytometry analysis. We further detected expression of apoptosis-related proteins. In the agomir group, the level of pro-apoptotic proteins such as cleaved-caspase-3, cleaved-PARP and Bad significantly reduced, while Bcl-2 and Bcl-2/Bax ratio increased. Opposite changes were observed in the antagomir group. Downstream targets of MLK3, JNK and c-jun, were also suppressed by miR-138. Our study demonstrates that up-regulation of miR-138 plays a protective role in myocardial adaptation to chronic hypoxia, which is mediated mainly by MLK3/JNK/c-jun signaling pathway.

HBx activates FasL and mediates HepG2 cell apoptosis through MLK3-MKK7-JNKs signal module

AIM

To investigate the possible mechanism by which hepatitis B virus X protein (HBx) mediates apoptosis of HepG2 cells.

METHODS

HBx expression vector pcDNA3.1-X was transfected into HepG2 cells to establish an HBx high-expression cellular model as pcDNA3.1-X transfected group. The pcDNA3.1-X and pSilencer3.1-shHBX (HBx antagonist) were cotransfected into HepG2 cells to establish an HBx low-expression model as RNAi group. Untransfected HepG2 cells and HepG2 cells transfected with negative control plasmid were used as controls. Apoptosis rate, the expression of Fas/FasL signaling pathway-related proteins and the phosphorylation levels of MLK3, MKK7 and JNKs, which are upstream molecules of death receptor pathways and belong to the family of mitogen-activated protein kinases (MAPKs), were measured in each group.

RESULTS

Compared with HepG2 cell group and RNAi group, apoptosis rate, the expression of Fas and FasL proteins, and the activation of MLK3, MKK7 and JNKs were increased in the pcDNA3.1-X transfected group. The activation of JNKs and expression of FasL protein were inhibited in the pcDNA3.1-X transfected group when treated with a known JNK inhibitor, SP600125. When authors treated pcDNA3.1-X transfected group with K252a, a known MLK3 inhibitor, the activation of MLK3, MKK7 and JNKs as well as expression of FasL protein was inhibited. Furthermore, cell apoptosis rate was also significantly declined in the presence of K252a in the pcDNA3.1-X transfected group.

CONCLUSION

HBx can induce HepG2 cell apoptosis via a novel active MLK3-MKK7-JNKs signaling module to upregulate FasL protein expression.