The Tumor Suppressor DAPK Is Reciprocally Regulated by Tyrosine Kinase Src and Phosphatase LAR

Death-associated protein kinase 1 as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly and represents a major clinical challenge in the ageing society. Neuropathological hallmarks of AD include neurofibrillary tangles composed of hyperphosphorylated tau, senile plaques derived from the deposition of amyloid-β (Aβ) peptides, brain atrophy induced by neuronal loss, and synaptic dysfunctions. Death-associated protein kinase 1 (DAPK1) is ubiquitously expressed in the central nervous system. Dysregulation of DAPK1 has been shown to contribute to various neurological diseases including AD, ischemic stroke and Parkinson's disease (PD). We have established an upstream effect of DAPK1 on Aβ and tau pathologies and neuronal apoptosis through kinase-mediated protein phosphorylation, supporting a causal role of DAPK1 in the pathophysiology of AD. In this review, we summarize current knowledge about how DAPK1 is involved in various AD pathological changes including tau hyperphosphorylation, Aβ deposition, neuronal cell death and synaptic degeneration. The underlying molecular mechanisms of DAPK1 dysregulation in AD are discussed. We also review the recent progress regarding the development of novel DAPK1 modulators and their potential applications in AD intervention. These findings substantiate DAPK1 as a novel therapeutic target for the development of multifunctional disease-modifying treatments for AD and other neurological disorders.

An integrative approach unveils a distal encounter site for rPTPε and phospho-Src complex formation

The structure determination of protein tyrosine phosphatase (PTP): phospho-protein complexes, which is essential to understand how specificity is achieved at the amino acid level, remains a significant challenge for protein crystallography and cryoEM due to the transient nature of binding interactions. Using rPTPεD1 and phospho-SrcKD as a model system, we have established an integrative workflow to address this problem, by means of which we generate a protein:phospho-protein complex model using predetermined protein structures, SAXS and pTyr-tailored MD simulations. Our model reveals transient protein-protein interactions between rPTPεD1 and phospho-SrcKD and is supported by three independent experimental validations. Measurements of the association rate between rPTPεD1 and phospho-SrcKD showed that mutations on the rPTPεD1: SrcKD complex interface disrupts these transient interactions, resulting in a reduction in protein-protein association rate and, eventually, phosphatase activity. This integrative approach is applicable to other PTP: phospho-protein complexes and the characterization of transient protein-protein interface interactions.

Ca2+ Signaling and Src Functions in Tumor Cells

Signaling by calcium ion (Ca2+) plays a prominent role in cell physiology, and these mechanisms are frequently altered in tumor cells. In this review, we consider the interplay of Ca2+ signaling and the functions of the proto-oncogene non-receptor tyrosine kinase c-Src in tumor cells, and the viral oncogenic variant v-Src in transformed cells. Also, other members of the Src-family kinases are considered in this context. The role of Ca2+ in the cell is frequently mediated by Ca2+-binding proteins, where the Ca2+-sensor protein calmodulin (CaM) plays a prominent, essential role in many cellular signaling pathways. Thus, we cover the available information on the role and direct interaction of CaM with c-Src and v-Src in cancerous cells, the phosphorylation of CaM by v-Src/c-Src, and the actions of different CaM-regulated Ser/Thr-protein kinases and the CaM-dependent phosphatase calcineurin on v-Src/c-Src. Finally, we mention some clinical implications of these systems to identify mechanisms that could be targeted for the therapeutic treatment of human cancers.

Targeting Death-Associated Protein Kinases for Treatment of Human Diseases: Recent Advances and Future Directions

The death-associated protein kinase (DAPK) family is a member of the calcium/calmodulin-regulated serine/threonine protein kinase family, and studies have shown that its role, as its name suggests, is mainly to regulate cell death. The DAPK family comprises five members, including DAPK1, DAPK2, DAPK3, DRAK1 and DRAK2, which show high homology in the common N-terminal kinase domain but differ in the extra-catalytic domain. Notably, previous research has suggested that the DAPK family plays an essential role in both the development and regulation of human diseases. However, only a few small-molecule inhibitors have been reported. In this Perspective, we mainly discuss the structure, biological function, and role of DAPKs in diseases and the currently discovered small-molecule inhibitors, providing valuable information for the development of the DAPK field.

Tumor Suppressor DAPK1 Catalyzes Adhesion Assembly on Rigid but Anoikis on Soft Matrices

Cancer cells normally grow on soft surfaces due to impaired mechanosensing of the extracellular matrix rigidity. Upon restoration of proper mechanosensing, cancer cells undergo apoptosis on soft surfaces (anoikis) like most normal cells. However, the link between mechanosensing and activation of anoikis is not clear. Here we show that death associated protein kinase 1 (DAPK1), a tumor suppressor that activates cell death, is directly linked to anoikis activation through rigidity sensing. We find that when rigidity sensing is decreased through inhibition of DAPK1 activity, cells are transformed for growth on soft matrices. Further, DAPK1 catalyzes matrix adhesion assembly and is part of adhesions on rigid surfaces. This pathway involves DAPK1 phosphorylation of tropomyosin1.1, the talin1 head domain, and tyrosine phosphorylation of DAPK1 by Src. On soft surfaces, DAPK1 rapidly dissociates from the adhesion complexes and activates apoptosis as catalyzed by PTPN12 activity and talin1 head. Thus, DAPK1 is important for adhesion assembly on rigid surfaces and the activation of anoikis on soft surfaces through its binding to rigidity-sensing modules.

Regulation of DAPK1 by Natural Products: An Important Target in Treatment of Stroke

Stroke is a sudden neurological disorder that occurs due to impaired blood flow to an area of the brain. Stroke can be caused by the blockage or rupture of a blood vessel in the brain, called ischemic stroke and hemorrhagic stroke, respectively. Stroke is more common in men than women. Atrial fibrillation, hypertension, kidney disease, high cholesterol and lipids, genetic predisposition, inactivity, poor nutrition, diabetes mellitus, family history and smoking are factors that increase the risk of stroke. Restoring blood flow by repositioning blocked arteries using thrombolytic agents or endovascular therapy are the most effective treatments for stroke. However, restoring circulation after thrombolysis can cause fatal edema or intracranial hemorrhage, and worsen brain damage in a process known as ischemia-reperfusion injury. Therefore, there is a pressing need to find and develop more effective treatments for stroke. In the past, the first choice of treatment was based on natural compounds. Natural compounds are able to reduce the symptoms and reduce various diseases including stroke that attract the attention of the pharmaceutical industry. Nowadays, as a result of the numerous studies carried out in the field of herbal medicine, many useful and valuable effects of plants have been identified. The death-associated protein kinase (DAPK) family is one of the vital families of serine/threonine kinases involved in the regulation of some biological functions in human cells. DAPK1 is the most studied kinase within the DAPKs family as it is involved in neuronal and recovery processes. Dysregulation of DAPK1 in the brain is involved in the developing neurological diseases such as stroke. Natural products can function in a variety of ways, including reducing cerebral edema, reducing brain endothelial cell death, and inhibiting TNFα and interleukin-1β (IL-1β) through regulating the DAPK1 signal against stroke. Due to the role of DAPK1 in neurological disorders, the aim of this article was to investigate the role of DAPK1 in stroke and its modulation by natural compounds.

Inorganic Phosphate (Pi) Signaling in Endothelial Cells: A Molecular Basis for Generation of Endothelial Microvesicles in Uraemic Cardiovascular Disease

Hyperphosphataemia increases cardiovascular mortality in patients with kidney disease. Direct effects of high inorganic phosphate (Pi) concentrations have previously been demonstrated on endothelial cells (ECs), including generation of procoagulant endothelial microvesicles (MVs). However, no mechanism directly sensing elevated intracellular Pi has ever been described in mammalian cells. Here, we investigated the hypothesis that direct inhibition by Pi of the phosphoprotein phosphatase PP2A fulfils this sensing role in ECs, culminating in cytoskeleton disruption and MV generation. ECs were treated with control (1 mM [Pi]) vs. high (2.5 mM [Pi]), a condition that drives actin stress fibre depletion and MV generation demonstrated by confocal microscopy of F-actin and NanoSight Nanoparticle tracking, respectively. Immuno-blotting demonstrated that high Pi increased p-Src, p-PP2A-C and p-DAPK-1 and decreased p-TPM-3. Pi at 100 μM directly inhibited PP2A catalytic activity. Inhibition of PP2A enhanced inhibitory phosphorylation of DAPK-1, leading to hypophosphorylation of Tropomyosin-3 at S284 and MV generation. p-Src is known to perform inhibitory phosphorylation on DAPK-1 but also on PP2A-C. However, PP2A-C can itself dephosphorylate (and therefore inhibit) p-Src. The direct inhibition of PP2A-C by Pi is, therefore, amplified by the feedback loop between PP2A-C and p-Src, resulting in further PP2A-C inhibition. These data demonstrated that PP2A/Src acts as a potent sensor and amplifier of Pi signals which can further signal through DAPK-1/Tropomyosin-3 to generate cytoskeleton disruption and generation of potentially pathological MVs.

Death-Associated Protein Kinase 1 as a Promising Drug Target in Cancer and Alzheimer's Disease

Background:

Death-Associated Protein Kinase 1 (DAPK1) plays an important role in apopto-sis, tumor suppression and neurodegeneration including Alzheimer’s Disease (AD).

Objective:

This review will describe the diverse roles of DAPK1 in the development of cancer and AD, and the current status of drug development targeting DAPK1-based therapies.

Methods:

Reports of DAPK1 regulation, function and substrates were analyzed using genetic DAPK1 manipulation and chemical DAPK1 modulators.

Results:

DAPK1 expression and activity are deregulated in cancer and AD. It is down-regulated and/or inactivated by multiple mechanisms in many human cancers, and elicits a protective effect to counteract numerous death stimuli in cancer, including activation of the master regulator Pin1. Moreover, loss of DAPK1 expression has correlated strongly with tumor recurrence and metastasis, suggesting that lack of sufficient functional DAPK1 might contribute to cancer. In contrast, DAPK1 is highly expressed in the brains of most human AD patients and has been identified as one of the genetic factors affecting suscepti-bility to late-onset AD. The absence of DAPK1 promotes efficient learning and better memory in mice and prevents the development of AD by acting on many key proteins including Pin1 and its downstream tar-gets tau and APP. Recent patents show that DAPK1 modulation might be used to treat both cancer and AD.

Conclusion:

DAPK1 plays a critical role in diverse physiological processes and importantly, its deregula-tion is implicated in the pathogenesis of either cancer or AD. Therefore, manipulating DAPK1 activity and/or expression may be a promising therapeutic option for cancer or AD.

The Role of Calmodulin in Tumor Cell Migration, Invasiveness, and Metastasis

Calmodulin (CaM) is the principal Ca²⁺ sensor protein in all eukaryotic cells, that upon binding to target proteins transduces signals encoded by global or subcellular-specific changes of Ca²⁺ concentration within the cell. The Ca²⁺/CaM complex as well as Ca²⁺-free CaM modulate the activity of a vast number of enzymes, channels, signaling, adaptor and structural proteins, and hence the functionality of implicated signaling pathways, which control multiple cellular functions. A basic and important cellular function controlled by CaM in various ways is cell motility. Here we discuss the role of CaM-dependent systems involved in cell migration, tumor cell invasiveness, and metastasis development. Emphasis is given to phosphorylation/dephosphorylation events catalyzed by myosin light-chain kinase, CaM-dependent kinase-II, as well as other CaM-dependent kinases, and the CaM-dependent phosphatase calcineurin. In addition, the role of the CaM-regulated small GTPases Rac1 and Cdc42 (cell division cycle protein 42) as well as CaM-binding adaptor/scaffold proteins such as Grb7 (growth factor receptor bound protein 7), IQGAP (IQ motif containing GTPase activating protein) and AKAP12 (A kinase anchoring protein 12) will be reviewed. CaM-regulated mechanisms in cancer cells responsible for their greater migratory capacity compared to non-malignant cells, invasion of adjacent normal tissues and their systemic dissemination will be discussed, including closely linked processes such as the epithelial–mesenchymal transition and the activation of metalloproteases. This review covers as well the role of CaM in establishing metastatic foci in distant organs. Finally, the use of CaM antagonists and other blocking techniques to downregulate CaM-dependent systems aimed at preventing cancer cell invasiveness and metastasis development will be outlined.

Cullin 3 and Its Role in Tumorigenesis

Cullin 3 (Cul3) family of ubiquitin ligases comprises three components, the RING finger protein RBX1, the Cul3 scaffold, and a Bric-a-brac/Tramtrack/Broad complex (BTB) protein. The BTB protein serves as a bridge to connect Cul3 to substrate and is functionally equivalent to the combination of substrate adaptor and linker in other Cullin complexes. Human genome encodes for ~180 BTB proteins, implying a broad spectrum of ubiquitination signals and substrate repertoire. Accordingly, Cul3 ubiquitin ligases are involved in diverse cellular processes, including cell division, differentiation, cytoskeleton remodeling, stress responses, and nerve cell functions. Emerging evidence has pointed to the prominent role of Cul3 ubiquitin ligases in cancer. This chapter will describe recent advances on the roles of Cul3 E3 ligase complexes in regulating various cancer hallmarks and therapeutic responses and the mutation/dysregulation of Cul3 substrate adaptors in cancer. In particular, we will focus on several extensively studied substrate adaptors, such as Keap1, SPOP, KLHL20, and LZTR1, and will also discuss other recently identified Cul3 adaptors with oncogenic or tumor-suppressive functions. We conclude that Cul3 ubiquitin ligases represent master regulators of human malignancies and highlight the importance of developing modulating agents for oncogenic/tumor-suppressive Cul3 E3 ligase complexes to prevent or intervene tumorigenesis.

Death-Associated Protein Kinase 1 Phosphorylation in Neuronal Cell Death and Neurodegenerative Disease

Regulated neuronal cell death plays an essential role in biological processes in normal physiology, including the development of the nervous system. However, the deregulation of neuronal apoptosis by various factors leads to neurodegenerative diseases such as ischemic stroke and Alzheimer’s disease (AD). Death-associated protein kinase 1 (DAPK1) is a calcium/calmodulin (Ca²⁺/CaM)-dependent serine/threonine (Ser/Thr) protein kinase that activates death signaling and regulates apoptotic neuronal cell death. Although DAPK1 is tightly regulated under physiological conditions, DAPK1 deregulation in the brain contributes to the development of neurological disorders. In this review, we describe the molecular mechanisms of DAPK1 regulation in neurons under various stresses. We also discuss the role of DAPK1 signaling in the phosphorylation-dependent and phosphorylation-independent regulation of its downstream targets in neuronal cell death. Moreover, we focus on the major impact of DAPK1 deregulation on the progression of neurodegenerative diseases and the development of drugs targeting DAPK1 for the treatment of diseases. Therefore, this review summarizes the DAPK1 phosphorylation signaling pathways in various neurodegenerative diseases.

Chromatin restriction by the nucleosome remodeler Mi-2β and functional interplay with lineage-specific transcription regulators control B-cell differentiation

Here, Yoshida et al. investigate the role of Mi-2β, a SNF-2-like nucleosome remodeler and key component of the nucleosome remodeling and histone deacetylase (NuRD) complex in early B cells. They found that the nucleosome remodeler Mi-2β promotes pre-B-cell differentiation by providing repression capabilities to distinct lineage-specific transcription factor-based regulatory networks.

Coordinated induction, but also repression, of genes are key to normal differentiation. Although the role of lineage-specific transcription regulators has been studied extensively, their functional integration with chromatin remodelers, one of the key enzymatic machineries that control chromatin accessibility, remains ill-defined. Here we investigate the role of Mi-2β, a SNF-2-like nucleosome remodeler and key component of the nucleosome remodeling and histone deacetylase (NuRD) complex in early B cells. Inactivation of Mi-2β arrested differentiation at the large pre-B-cell stage and caused derepression of cell adhesion and cell migration signaling factors by increasing chromatin access at poised enhancers and chromosome architectural elements. Mi-2β also supported IL-7R signaling, survival, and proliferation by repressing negative effectors of this pathway. Importantly, overexpression of Bcl2, a mitochondrial prosurvival gene and target of IL-7R signaling, partly rescued the differentiation block caused by Mi-2β loss. Mi-2β stably associated with chromatin sites that harbor binding motifs for IKAROS and EBF1 and physically associated with these transcription factors both on and off chromatin. Notably, Mi-2β shared loss-of-function cellular and molecular phenotypes with IKAROS and EBF1, albeit in a distinct fashion. Thus, the nucleosome remodeler Mi-2β promotes pre-B-cell differentiation by providing repression capabilities to distinct lineage-specific transcription factor-based regulatory networks.

Profiling of gallbladder carcinoma reveals distinct miRNA profiles and activation of STAT1 by the tumor suppressive miRNA-145-5p

Gallbladder carcinoma (GBC) is a biliary tract cancer with few treatment options and poor prognosis. Radical surgery is the only potentially curative treatment option but most patients diagnosed with GBC are unresectable. Thus, there is a great need for the development of new treatment options including targeted therapy. Here, we aimed at identifying deregulated miRNAs and affected pathways involved in GBC development and progression. We performed global miRNA profiling of 40 GBC and 8 normal gallbladder tissues and identified large differences with 30% of miRNAs being differentially expressed (false discovery rate: FDR < 0.001). We found 24 miRNAs to be differentially regulated in GBC with poor outcome (p < 0.05) of which miR-145-5p was the most downregulated miRNA. Overexpression of miR-145-5p significantly reduced cell proliferation and colony formation. Gene expression analysis of cells expressing miR-145-5p mimics revealed activation of the Signal transducer and activator of transcription 1 (STAT1) signaling pathway which is mainly tumor suppressive. Furthermore, the activation of STAT1 by miR-145-5p was specifically observed in gallbladder carcinoma and cholangiocarcinoma but not in hepatocellular carcinoma cells. The Protein Tyrosine Phosphatase Receptor Type F (PTPRF) is downregulated upon miR-145 expression and may be involved in STAT1 regulation. In addition, we found that the STAT1-regulated protein IRF7 is downregulated in GBC compared to normal gallbladder tissue and low IRF7 expression is associated with significantly lower overall survival of GBC patients. Thus, this study identified GBC patient subgroups and provides new mechanistic insights in the tumor suppressive function of miR-145-5p leading to activation of STAT1 signaling.

Tumor suppressive protein phosphatases in human cancer: Emerging targets for therapeutic intervention and tumor stratification

Aberrant protein phosphorylation is one of the hallmarks of cancer cells, and in many cases a prerequisite to sustain tumor development and progression. Like protein kinases, protein phosphatases are key regulators of cell signaling. However, their contribution to aberrant signaling in cancer cells is overall less well appreciated, and therefore, their clinical potential remains largely unexploited. In this review, we provide an overview of tumor suppressive protein phosphatases in human cancer. Along their mechanisms of inactivation in defined cancer contexts, we give an overview of their functional roles in diverse signaling pathways that contribute to their tumor suppressive abilities. Finally, we discuss their emerging roles as predictive or prognostic markers, their potential as synthetic lethality targets, and the current feasibility of their reactivation with pharmacologic compounds as promising new cancer therapies. We conclude that their inclusion in clinical practice has obvious potential to significantly improve therapeutic outcome in various ways, and should now definitely be pushed forward.

Restoration of DAP Kinase Tumor Suppressor Function: A Therapeutic Strategy to Selectively Induce Apoptosis in Cancer Cells Using Immunokinase Fusion Proteins

Targeted cancer immunotherapy is designed to selectively eliminate tumor cells without harming the surrounding healthy tissues. The death-associated protein kinases (DAPk) are a family of proapoptotic proteins that play a vital role in the regulation of cellular process and have been identified as positive mediators of apoptosis via extrinsic and intrinsic death-regulating signaling pathways. Tumor suppressor activities have been shown for DAPk1 and DAPk2 and they are downregulated in e.g., Hodgkin's (HL) and B cell lymphoma (CLL), respectively. Here, we review a targeted therapeutic approach which involves reconstitution of DAPks by the generation of immunokinase fusion proteins. These recombinant proteins consist of a disease-specific ligand fused to a modified version of DAPk1 or DAPk2. HL was targeted via CD30 and B-CLL via CD22 cell surface antigens.

DAPK1 as an independent prognostic marker in liver cancer

The death-associated protein kinase 1 (DAPK1) can act as an oncogene or a tumor suppressor gene depending on the cellular context as well as external stimuli. Our study aims to investigate the prognostic significance of DAPK1 in liver cancer in both mRNA and protein levels. The mRNA expression of DAPK1 was extracted from the Gene Expression Omnibus database in three independent liver cancer datasets while protein expression of DAPK1 was detected by immunohistochemistry in our Chinese liver cancer patient cohort. The associations between DAPK1 expression and clinical characteristics were tested. DAPK1 mRNA expression was down-regulated in liver cancer. Low levels of DAPK1 mRNA were associated with shorter survival in a liver cancer patient cohort (n = 115; p = 0.041), while negative staining of DAPK1 protein was significantly correlated with shorter time to progression (p = 0.002) and overall survival (p = 0.02). DAPK1 was an independent prognostic marker for both time to progression and overall survival by multivariate analysis. Liver cancer with the b-catenin mutation has a lower DAPK1 expression, suggesting that DAPK1 may be regulated under the b-catenin pathway. In addition, we also identified genes that are co-regulated with DAPK1. DAPK1 expression was positively correlated with IRF2, IL7R, PCOLCE and ZBTB16, and negatively correlated with SLC16A3 in both liver cancer datasets. Among these genes, PCOLCE and ZBTB16 were significantly down-regulated, while SLC16A3 was significantly upregulated in liver cancer. By using connectivity mapping of these co-regulated genes, we have identified amcinonide and sulpiride as potential small molecules that could potentially reverse DAPK1/PCOLCE/ZBTB16/SLC16A3 expression. Our study demonstrated for the first time that both DAPK1 mRNA and protein expression levels are important prognostic markers in liver cancer, and have identified genes that may contribute to DAPK1-mediated liver carcinogenesis.

Ubiquitination of tumor suppressor PML regulates prometastatic and immunosuppressive tumor microenvironment

The tumor microenvironment plays an important role in tumor growth and metastasis. However, the mechanism by which tumor cells regulate the cell and non-cell constituents of surrounding stroma remains incompletely understood. Promyelocytic leukemia (PML) is a pleiotropic tumor suppressor, but its role in tumor microenvironment regulation is poorly characterized. PML is frequently downregulated in many cancer types, including lung cancer. Here, we identify a PML ubiquitination pathway that is mediated by WD repeat 4-containing cullin-RING ubiquitin ligase 4 (CRL4WDR4). Clinically, this PML degradation pathway is hyperactivated in lung cancer and correlates with poor prognosis. The WDR4/PML axis induces a set of cell-surface or secreted factors, including CD73, urokinase-type plasminogen activator receptor (uPAR), and serum amyloid A2 (SAA2), which elicit paracrine effects to stimulate migration, invasion, and metastasis in multiple lung cancer models. In xenograft and genetically engineered mouse models, the WDR4/PML axis elevates intratumoral Tregs and M2-like macrophages and reduces CD8+ T cells to promote lung tumor growth. These immunosuppressive effects were all reversed by CD73 blockade. Our study identifies WDR4 as an oncoprotein that negatively regulates PML via ubiquitination to promote lung cancer progression by fostering an immunosuppressive and prometastatic tumor microenvironment, suggesting the potential of immune-modulatory approaches for treating lung cancer with aberrant PML degradation.

Cullin 3 Ubiquitin Ligases in Cancer Biology: Functions and Therapeutic Implications

Cullin-RING ubiquitin ligases are the largest E3 ligase family in eukaryotes and are multiprotein complexes. In these complexes, the Cullin protein serves as a scaffold to connect two functional modules of the ligases, the catalytic subunit and substrate-binding subunit. To date, eight members of the Cullin family proteins have been identified. In the Cul3 ubiquitin ligases, Bric-a-brac/Tramtrack/Broad complex (BTB) domain-containing proteins function as a bridge to connect Cul3 and substrates. While the BTB domain is responsible for Cul3 binding, these proteins usually contain an additional domain for substrate interaction, such as MATH, kelch, Zn finger, and PAM, Highwire, and RPM-1 (PHR domain). With the existence of a large number of BTB proteins in human, the Cul3 ubiquitin ligases ubiquitinate a wide range of substrates involving in diverse cellular functions. In this review, we will discuss recent advances on the functions of Cul3 ubiquitin ligases in cancer development, progression, and therapeutic response and the dysregulation of Cul3-mediated ubiquitination events in human malignancies. In particular, we will focus on three Cul3 substrate adaptors, kelch-like ECH-associated protein (Keap1), kelch-like family member 20 (KLHL20), and speckle type BTB/POZ protein (SPOP), with the intent to highlight novel targets in cancer therapy.

Cul3-KLHL20 ubiquitin ligase: physiological functions, stress responses, and disease implications

Cullin-RING ubiquitin ligases are the largest Ubiquitin ligase family in eukaryotes and are multi-protein complexes. In these complexes, the Cullin protein serves as a scaffold to connect two functional modules of the ligases, the catalytic subunit and substrate-binding subunit. KLHL20 is a substrate-binding subunit of Cullin3 (Cul3) ubiquitin ligase. Recent studies have identified a number of substrates of KLHL20-based ubiquitin ligase. Through ubiquitination of these substrates, KLHL20 elicits diverse cellular functions, some of which are associated with human diseases. Furthermore, the functions, subcellular localizations, and expression of KLHL20 are regulated by several physiological and stressed signals, which allow KLHL20 to preferentially act on certain substrates to response to these signals. Here, we provide a summary of the functions and regulations of KLHL20 in several physiological processes and stress responses and its disease implications.

LAR protein tyrosine phosphatase regulates focal adhesions via CDK1

Focal adhesions are complex multi-molecular structures that link the actin cytoskeleton to the extracellular matrix via integrin adhesion receptors and play a key role in regulation of many cellular functions. LAR is a receptor protein tyrosine phosphatase that regulates PDGF signalling and localises to focal adhesions. We have observed that loss of LAR phosphatase activity in mouse embryonic fibroblasts results in reduced numbers of focal adhesions and decreased adhesion to fibronectin. To understand how LAR regulates cell adhesion we used phosphoproteomic data, comparing global phosphorylation events in wild type and LAR phosphatase-deficient cells, to analyse differential kinase activity. Kinase prediction analysis of LAR-regulated phosphosites identified a node of cytoskeleton- and adhesion-related proteins centred on cyclin-dependent kinase-1 (CDK1). We found that loss of LAR activity resulted in reduced activity of CDK1, and that CDK1 activity was required for LAR-mediated focal adhesion complex formation. We also established that LAR regulates CDK1 activity via c-Abl and PKB/Akt. In summary, we have identified a novel role for a receptor protein tyrosine phosphatase in regulating CDK1 activity and hence cell adhesion to the extracellular matrix.

USP11 regulates PML stability to control Notch-induced malignancy in brain tumours

The promyelocytic leukaemia (PML) protein controls multiple tumour suppressive functions and is downregulated in diverse types of human cancers through incompletely characterized post-translational mechanisms. Here we identify USP11 as a PML regulator by RNAi screening. USP11 deubiquitinates and stabilizes PML, thereby counteracting the functions of PML ubiquitin ligases RNF4 and the KLHL20-Cul3 (Cullin 3)-Roc1 complex. We find that USP11 is transcriptionally repressed through a Notch/Hey1-dependent mechanism, leading to PML destabilization. In human glioma, Hey1 upregulation correlates with USP11 and PML downregulation and with high-grade malignancy. The Notch/Hey1-induced downregulation of USP11 and PML not only confers multiple malignant characteristics of aggressive glioma, including proliferation, invasiveness and tumour growth in an orthotopic mouse model, but also potentiates self-renewal, tumour-forming capacity and therapeutic resistance of patient-derived glioma-initiating cells. Our study uncovers a PML degradation mechanism through Notch/Hey1-induced repression of the PML deubiquitinase USP11 and suggests an important role for this pathway in brain tumour pathogenesis.

Novel Cancer Therapeutics with Allosteric Modulation of the Mitochondrial C-Raf-DAPK Complex by Raf Inhibitor Combination Therapy

Mitochondria are the powerhouses of cells. Mitochondrial C-Raf is a potential cancer therapeutic target, as it regulates mitochondrial function and is localized to the mitochondria by its N-terminal domain. However, Raf inhibitor monotherapy can induce S338 phosphorylation of C-Raf (pC-Raf(S338)) and impede therapy. This study identified the interaction of C-Raf with S308 phosphorylated DAPK (pDAPK(S308)), which together became colocalized in the mitochondria to facilitate mitochondrial remodeling. Combined use of the Raf inhibitors sorafenib and GW5074 had synergistic anticancer effects in vitro and in vivo, but targeted mitochondrial function, rather than the canonical Raf signaling pathway. C-Raf depletion in knockout MEF(C-Raf-/-) or siRNA knockdown ACHN renal cancer cells abrogated the cytotoxicity of combination therapy. Crystal structure simulation showed that GW5074 bound to C-Raf and induced a C-Raf conformational change that enhanced sorafenib-binding affinity. In the presence of pDAPK(S308), this drug-target interaction compromised the mitochondrial targeting effect of the N-terminal domain of C-Raf, which induced two-hit damages to cancer cells. First, combination therapy facilitated pC-Raf(S338) and pDAPK(S308) translocation from mitochondria to cytoplasm, leading to mitochondrial dysfunction and reactive oxygen species (ROS) generation. Second, ROS facilitated PP2A-mediated dephosphorylation of pDAPK(S308) to DAPK. PP2A then dissociated from the C-Raf-DAPK complex and induced profound cancer cell death. Increased pDAPK(S308) modification was also observed in renal cancer tissues, which correlated with poor disease-free survival and poor overall survival in renal cancer patients. Besides mediating the anticancer effect, pDAPK(S308) may serve as a predictive biomarker for Raf inhibitors combination therapy, suggesting an ideal preclinical model that is worthy of clinical translation.

Death-associated protein kinase: A molecule with functional antagonistic duality and a potential role in inflammatory bowel disease (Review)

The cytoskeleton-associated serine/threonine kinase death-associated protein kinase (DAPK) has been described as a cancer gene chameleon with functional antagonistic duality in a cell type and context specific manner. The broad range of interaction partners and substrates link DAPK to inflammatory processes especially in the gut. Herein we summarize our knowledge on the role of DAPK in different cell types that play a role under inflammatory conditions in the gut. Besides some promising experimental data suggesting DAPK as an interesting drug target in inflammatory bowel disease there are many open questions regarding direct evidence for a role of DAPK in intestinal inflammation.

The DAXX co-repressor is directly recruited to active regulatory elements genome-wide to regulate autophagy programs in a model of human prostate cancer

While carcinoma of the prostate is the second most common cause of cancer death in the US, current methods and markers used to predict prostate cancer (PCa) outcome are inadequate. This study was aimed at understanding the genome-wide binding and regulatory role of the DAXX transcriptional repressor, recently implicated in PCa. ChIP-Seq analysis of genome-wide distribution of DAXX in PC3 cells revealed over 59,000 DAXX binding sites, found at regulatory enhancers and promoters. ChIP-Seq analysis of DNA methyltransferase 1 (DNMT1), which is a key epigenetic partner for DAXX repression, revealed that DNMT1 binding was restricted to a small number of DAXX sites. DNMT1 and DAXX bound close to transcriptional activator motifs. DNMT1 sites were found to be dependent on DAXX for recruitment by analyzing DNMT1 ChIP-Seq following DAXX knockdown (K/D), corroborating previous findings that DAXX recruits DNMT1 to repress its target genes. Massively parallel RNA sequencing (RNA-Seq) was used to compare the transcriptomes of WT and DAXX K/D PC3 cells. Genes induced by DAXX K/D included those involved in autophagy, and DAXX ChIP-Seq peaks were found close to the transcription start sites (TSS) of autophagy genes, implying they are more likely to be regulated by DAXX. In conclusion, DAXX binds active regulatory elements and co-localizes with DNMT1 in the prostate cancer genome. Given DAXX's putative regulatory role in autophagy, future studies may consider DAXX as a candidate marker and therapeutic target for prostate cancer.

Clinical significance of DAPK promoter hypermethylation in lung cancer: a meta-analysis

Death-associated protein kinase 1 (DAPK) is an important serine/threonine kinase involved in various cellular processes, including apoptosis, autophagy, and inflammation. DAPK expression and activity are deregulated in a variety of diseases including cancer. Methylation of the DAPK gene is common in many types of cancer and can lead to loss of DAPK expression. However, the association between DAPK promoter hypermethylation and the clinicopathological significance of lung cancer remains unclear. In this study, we searched the MEDLINE, PubMed, Web of Science, and Scopus databases, systematically investigated the studies of DAPK promoter hypermethylation in lung cancer and quantified the association between DAPK promoter hypermethylation and its clinicopathological significance by meta-analysis. We observed that the frequency of DAPK methylation was significantly higher in lung cancer than in non-malignant lung tissues (odds ratio 6.02, 95% confidence interval 3.17-11.42, P<0.00001). The pooled results also showed the presence of a prognostic impact of DAPK gene methylation in lung cancer patients (odds ratio 3.63, 95% confidence interval 1.09-12.06, P=0.04). In addition, we summarized these findings and discuss tumor suppressor function, clinicopathological significance, and potential drug targeting of DAPK in lung cancer.

Mitotic arrest and slippage induced by pharmacological inhibition of Polo-like kinase 1

Exposure to drugs that interfere with microtubule dynamics block cell cycle progression at mitosis by prolonged activation of the spindle assembly checkpoint (SAC). Cells can evade mitotic arrest and proceed to interphase without chromosome segregation by a process termed mitotic slippage that involves Cyclin B1 degradation without checkpoint inactivation. Here, we explored the cellular response to small-molecule inhibitors of Polo-like kinase 1 (Plk1), an important regulator of cell division. We found that the clinical Plk1 inhibitors BI 2536 and BI 6727, both unexpectedly, induced a dose-dependent cellular drug response: While mitotic arrest was induced in cancer cell lines and primary non-transformed cells across the entire range of concentrations tested, only high concentrations seemed to promote mitotic slippage. Since this observation contrasts with the effects expected from studies reporting RNAi-mediated Plk1 depletion in cancer cells, we wondered whether both ATP-competitive inhibitors target unknown kinases that are involved in signaling from the spindle assembly checkpoint (SAC) and might contribute to the mitotic slippage. A chemical proteomics approach used to profile the selectivity of both inhibitors revealed that SAC kinases are not targeted directly. Still, the activities of Cdk1/Cyclin B1 and Aurora B, which plays important roles in the error correction of false microtubule-kinetochore attachments and in checkpoint signaling, were shown to be downregulated at high inhibitor concentrations. Our data suggest that the inhibition of Plk1 activity below a certain threshold influences Aurora B activity via reduced phosphorylation of Fox M1 and Survivin leading to diminished levels of Aurora B protein and alteration of its subcellular localization. Within the spectrum of SAC proteins that are degraded during mitotic slippage, the degradation of Cyclin B1 and the downregulation of Aurora B activity by Plk1 inhibition seem to be critical promoters of mitotic slippage. The results indicate that careful dose-finding studies in cancer trials are necessary to limit or even prevent mitotic slippage, which could be associated with improved cancer cell survival.

Receptor-type protein tyrosine phosphatases in cancer

Protein tyrosine phosphatases (PTPs) play an important role in regulating cell signaling events in coordination with tyrosine kinases to control cell proliferation, apoptosis, survival, migration, and invasion. Receptor-type protein tyrosine phosphatases (PTPRs) are a subgroup of PTPs that share a transmembrane domain with resulting similarities in function and target specificity. In this review, we summarize genetic and epigenetic alterations including mutation, deletion, amplification, and promoter methylation of PTPRs in cancer and consider the consequences of PTPR alterations in different types of cancers. We also summarize recent developments using PTPRs as prognostic or predictive biomarkers and/or direct targets. Increased understanding of the role of PTPRs in cancer may provide opportunities to improve therapeutic approaches.

SCP phosphatases suppress renal cell carcinoma by stabilizing PML and inhibiting mTOR/HIF signaling

The tumor-suppressor protein promyelocytic leukemia (PML) is aberrantly degraded in multiple types of human cancers through mechanisms that are incompletely understood. Here, we show that the phosphatase SCP1 and its isoforms SCP2/3 dephosphorylate PML at S518, thereby blocking PML ubiquitination and degradation mediated by the prolyl isomerase Pin1 and the ubiquitin ligase KLHL20. Clinically, SCP1 and SCP3 are downregulated in clear cell renal cell carcinoma (ccRCC) and these events correlated with PMLS518 phosphorylation, PML turnover, and high-grade tumors. Restoring SCP1-mediated PML stabilization not only inhibited malignant features of ccRCC, including proliferation, migration, invasion, tumor growth, and tumor angiogenesis, but also suppressed the mTOR-HIF pathway. Furthermore, blocking PML degradation in ccRCC by SCP1 overexpression or Pin1 inhibition enhanced the tumor-suppressive effects of the mTOR inhibitor temsirolimus. Taken together, our results define a novel pathway of PML degradation in ccRCC that involves SCP downregulation, revealing contributions of this pathway to ccRCC progression and offering a mechanistic rationale for combination therapies that jointly target PML degradation and mTOR inhibition for ccRCC treatment.

Regulation of inflammation by DAPK

Death-associated protein kinase (DAPK) is a tumor suppressor and negatively regulates several activation signals. Consistent with its potential anti-inflammatory activity, DAPK promotes the formation of IFN-γ-activated inhibitor of translation (GAIT) complex that suppresses the translation of selected inflammatory genes. DAPK has been found to inhibit tumor necrosis factor-α (TNF-α)- or lipopolysaccharides (LPS)-induced NF-κB activation and pro-inflammatory cytokine expression. Inflammation is always associated with T cell activation, while DAPK attenuates T cell activation by a selective suppression in T cell receptor-triggered NF-κB activation. Recent studies, however, also reveal a contribution of DAPK to pro-inflammatory processes. DAPK is shown to mediate pro-inflammatory signaling downstream of TNF-α, LPS, IL-17, or IL-32. In addition, DAPK is required for the full formation of NLRP3 inflammasome, essential for the generation of IL-1β and IL-18. These results suggest the complicated role of DAPK in the regulation of inflammation that is likely dependent on cell types and environmental cues.

The DAPK family: a structure-function analysis

DAP-kinase (DAPK) is the founding member of a family of highly related, death associated Ser/Thr kinases that belongs to the calmodulin (CaM)-regulated kinase superfamily. The family includes DRP-1 and ZIP-kinase (ZIPK), both of which share significant homology within the common N-terminal kinase domain, but differ in their extra-catalytic domains. Both DAPK and DRP-1 possess a conserved CaM autoregulatory domain, and are regulated by calcium-activated CaM and by an inhibitory auto-phosphorylation within the domain. ZIPK's activity is independent of CaM but can be activated by DAPK. The three kinases share some common functions and substrates, such as induction of autophagy and phosphorylation of myosin regulatory light chain leading to membrane blebbing. Furthermore, all can function as tumor suppressors. However, they also each possess unique functions and intracellular localizations, which may arise from the divergence in structure in their respective C-termini. In this review we will introduce the DAPK family, and present a structure/function analysis for each individual member, and for the family as a whole. Emphasis will be placed on the various domains, and how they mediate interactions with additional proteins and/or regulation of kinase function.

The DAP-kinase interactome

DAP-kinase (DAPK) is a Ca(2+)/calmodulin regulated Ser/Thr kinase that activates a diverse range of cellular activities. It is subject to multiple layers of regulation involving both intramolecular signaling, and interactions with additional proteins, including other kinases and phosphatases. Its protein stability is modulated by at least three distinct ubiquitin-dependent systems. Like many kinases, DAPK participates in several signaling cascades, by phosphorylating additional kinases such as ZIP-kinase and protein kinase D (PKD), or Pin1, a phospho-directed peptidyl-prolyl isomerase that regulates the function of many phosphorylated proteins. Other substrate targets have more direct cellular effects; for example, phosphorylation of the myosin II regulatory chain and tropomyosin mediate some of DAPK's cytoskeletal functions, including membrane blebbing during cell death and cell motility. DAPK induces distinct death pathways of apoptosis, autophagy and programmed necrosis. Among the substrates implicated in these processes, phosphorylation of PKD, Beclin 1, and the NMDA receptor has been reported. Interestingly, not all cellular effects are mediated by DAPK's catalytic activity. For example, by virtue of protein-protein interactions alone, DAPK activates pyruvate kinase isoform M2, the microtubule affinity regulating kinases and inflammasome protein NLRP3, to promote glycolysis, influence microtubule dynamics, and enhance interleukin-1β production, respectively. In addition, a number of other substrates and interacting proteins have been identified, the physiological significance of which has not yet been established. All of these substrates, effectors and regulators together comprise the DAPK interactome. By presenting the components of the interactome network, this review will clarify both the mechanisms by which DAPK function is regulated, and by which it mediates its various cellular effects.

TGF-β: duality of function between tumor prevention and carcinogenesis

Several mechanisms underlying tumor progression have remained elusive, particularly in relation to transforming growth factor beta (TGF-β). Although TGF-β initially inhibits epithelial growth, it appears to promote the progression of advanced tumors. Defects in normal TGF-β pathways partially explain this paradox, which can lead to a cascade of downstream events that drive multiple oncogenic pathways, manifesting as several key features of tumorigenesis (uncontrolled proliferation, loss of apoptosis, epithelial-to-mesenchymal transition, sustained angiogenesis, evasion of immune surveillance, and metastasis). Understanding the mechanisms of TGF-β dysregulation will likely reveal novel points of convergence between TGF-β and other pathways that can be specifically targeted for therapy.

DAPK plays an important role in panobinostat-induced autophagy and commits cells to apoptosis under autophagy deficient conditions

The histone deacetylase inhibitor (HDACi) LBH589 has been verified as an effective anticancer agent. The identification and characterization of new targets for LBH589 action would further enhance our understanding of the molecular mechanisms involved in HDACi therapy. The role of the tumor suppressor death-associated protein kinase (DAPK) in LBH589-induced cytotoxicity has not been investigated to date. Stable DAPK knockdown (shRNA) and DAPK overexpressing (DAPK+++) cell lines were generated from HCT116 wildtype colon cancer cells. LBH589 inhibited cell proliferation, reduced the long-term survival, and up-regulated and activated DAPK in colorectal cancer cells. Moreover, LBH589 significantly suppressed the growth of colon tumor xenografts and in accordance with the in vitro studies, increased DAPK levels were detected immunohistochemically. LBH589 induced a DAPK-dependent autophagy as assessed by punctuate accumulation of LC3-II, the formation of acidic vesicular organelles, and degradation of p62 protein. LBH589-induced autophagy seems to be predominantly caused by DAPK protein interactions than by its kinase activity. Caspase inhibitor zVAD increased autophagosome formation, decreased the cleavage of caspase 3 and PARP but didn't rescue the cells from LBH589-induced cell death in crystal violet staining suggesting both caspase-dependent as well as caspase-independent apoptosis pathways. Pre-treatment with the autophagy inhibitor Bafilomycin A1 caused caspase 3-mediated apoptosis in a DAPK-dependent manner. Altogether our data suggest that DAPK induces autophagy in response to HDACi-treatment. In autophagy deficient cells, DAPK plays an essential role in committing cells to HDACi-induced apoptosis.

CCN2 inhibits lung cancer metastasis through promoting DAPK-dependent anoikis and inducing EGFR degradation

CCN family protein 2 (CCN2), also known as connective tissue growth factor, is a secreting protein that modulates multiple cellular events. We previously demonstrated the metastasis-suppressive effect of CCN2 in lung cancer cells. In this study, we investigate the role of CCN2 in anoikis, a form of programmed cell death that is critical in suppressing cancer metastasis. CCN2 binds to the epidermal growth factor receptor (EGFR) and triggers ubiquitination by inhibiting the formation of the β-pix/Cbl complex, resulting in the degradation of EGFR. Binding of CCN2 to EGFR suppresses the phosphorylation of c-Src and extracellular signal-regulated kinase but increases the expression of death-associated protein kinase, which leads to anoikis. Overall, our findings provide evidence validating the use of CCN2 as an anti-metastatic therapy in lung cancer patients, and prospect a potential therapeutic synergy between CCN2 and the anti-EGFR antibody for the treatment of lung cancer.

miR-103/107 promote metastasis of colorectal cancer by targeting the metastasis suppressors DAPK and KLF4

Metastasis is the major cause of poor prognosis in colorectal cancer (CRC), and increasing evidence supports the contribution of miRNAs to cancer progression. Here, we found that high expression of miR-103 and miR-107 (miR-103/107) was associated with metastasis potential of CRC cell lines and poor prognosis in patients with CRC. We showed that miR-103/107 targeted the known metastasis suppressors death-associated protein kinase (DAPK) and Krüppel-like factor 4 (KLF4) in CRC cells, resulting in increased cell motility and cell-matrix adhesion and decreased cell-cell adhesion and epithelial marker expression. miR-103/107 expression was increased in the presence of hypoxia, thereby potentiating DAPK and KLF4 downregulation and hypoxia-induced motility and invasiveness. In mouse models of CRC, miR-103/107 overexpression potentiated local invasion and liver metastasis effects, which were suppressed by reexpression of DAPK or KLF4. miR-103/107-mediated downregulation of DAPK and KLF4 also enabled the colonization of CRC cells at a metastatic site. Clinically, the signature of a miR-103/107 high, DAPK low, and KLF4 low expression profile correlated with the extent of lymph node and distant metastasis in patients with CRC and served as a prognostic marker for metastasis recurrence and poor survival. Our findings therefore indicate that miR-103/107-mediated repression of DAPK and KLF4 promotes metastasis in CRC, and this regulatory circuit may contribute in part to hypoxia-stimulated tumor metastasis. Strategies that disrupt this regulation might be developed to block CRC metastasis.

Apoptosis signaling kinases: from stress response to health outcomes

Apoptosis is a highly regulated process essential for the development and homeostasis of multicellular organisms. Whereas caspases, a large family of intracellular cysteine proteases, play central roles in the execution of apoptosis, other proapoptotic and antiapoptotic regulators such as the members of the Bcl-2 family are also critically involved in the regulation of apoptosis. A large body of evidence has revealed that a number of protein kinases are among such regulators and regulate cellular sensitivity to various proapoptotic signals at multiple steps in apoptosis. However, recent progress in the analysis of these apoptosis signaling kinases demonstrates that they generally act as crucial regulators of diverse cellular responses to a wide variety of stressors, beyond their roles in apoptosis regulation. In this review, we have cataloged apoptosis signaling kinases involved in cellular stress responses on the basis of their ability to induce apoptosis and discuss their roles in stress responses with particular emphasis on health outcomes upon their dysregulation.

Latent membrane protein 1 of Epstein-Barr virus regulates death-associated protein kinase 1 in lymphoblastoid cell line

The Epstein-Barr virus (EBV) infects and transforms primary B cells into lymphoblastoid cell lines (LCLs). We observed death-associated protein kinase 1 (DAPK1) upregulation in B cells following EBV infection and high DAPK1 levels in LCLs. DAPK1 participates in several apoptosis-inducing pathways, yet DAPK1 expression increased during B cell transformation. Data from LMP1 overexpression in LCLs and HeLa cells and from knocked down LMP1 in LCLs suggest LMP1 regulation of DAPK1 expression. We observed NF-κB signaling in DAPK1 upregulation by LMP1 with CTAR deletion mutants failing to induce DAPK1 expression and with Bay11 blocking DAPK1 expression. DAPK1 is inactive in LCLs due to insufficient stimuli, and is not regulated by Ser308 phosphorylation. However, DAPK1 in LCLs is functional, as evidenced by its quick mediation of cell death following UV or H(2)O(2) exposure, and increased survival among LCLs knocked down with DAPK. DAPK roles in EBV-infected B cells remain to be identified.

DAPK activates MARK1/2 to regulate microtubule assembly, neuronal differentiation, and tau toxicity

Death-associated protein kinase (DAPK) is a key player in several modes of neuronal death/injury and has been implicated in the late-onset Alzheimer's disease (AD). DAPK promotes cell death partly through its effect on regulating actin cytoskeletons. In this study, we report that DAPK inhibits microtubule (MT) assembly by activating MARK/PAR-1 family kinases MARK1/2, which destabilize MT by phosphorylating tau and related MAP2/4. DAPK death domain, but not catalytic activity, is responsible for this activation by binding to MARK1/2 spacer region, thereby disrupting an intramolecular interaction that inhibits MARK1/2. Accordingly, DAPK(-/-) mice brain displays a reduction of tau phosphorylation and DAPK enhances the effect of MARK2 on regulating polarized neurite outgrowth. Using a well-characterized Drosophila model of tauopathy, we show that DAPK exerts an effect in part through MARK Drosophila ortholog PAR-1 to induce rough eye and loss of photoreceptor neurons. Furthermore, DAPK enhances tau toxicity through a PAR-1 phosphorylation-dependent mechanism. Together, our study reveals a novel mechanism of MARK activation, uncovers DAPK functions in modulating MT assembly and neuronal differentiation, and provides a molecular link of DAPK to tau phosphorylation, an event associated with AD pathology.

Testicular lumicrine factors regulate ERK, STAT, and NFKB pathways in the initial segment of the rat epididymis to prevent apoptosis

The initial segment of the epididymis is vital for male fertility; therefore, it is important to understand the mechanisms that regulate this important region. Deprival of testicular luminal fluid factors/lumicrine factors from the epididymis results in a wave of apoptosis in the initial segment. In this study, a combination of protein array and microarray analyses was used to examine the early changes in downstream signal transduction pathways following loss of lumicrine factors. We discovered the following cascade of events leading to the loss of protection and eventual apoptosis: in the first 6 h after loss of lumicrine factors, down-regulation of the ERK pathway components was observed at the mRNA expression and protein activity levels. Microarray analysis revealed that mRNA levels of several key components of the ERK pathway, Dusp6, Dusp5, and Etv5, decreased sharply, while the analysis from the protein array revealed a decline in the activities of MAP2K1/2 and MAPK1. Immunostaining of phospho-MAPK3/1 indicated that down-regulation of the ERK pathway was specific to the epithelial cells of the initial segment. Subsequently, after 12 h of loss of lumicrine factors, levels of mRNA expression of STAT and NFKB pathway components increased, mRNA levels of several genes encoding cell cycle inhibitors increased, and levels of protein expression of several proapoptotic phosphatases increased. Finally, after 18 h of loss of protection from lumicrine factors, apoptosis was observed. In conclusion, testicular lumicrine factors protect the cells of the initial segment by activating the ERK pathway, repressing STAT and NFKB pathways, and thereby preventing apoptosis.

Structures of parasitic CDPK domains point to a common mechanism of activation

We recently determined the first structures of inactivated and calcium-activated calcium-dependent protein kinases (CDPKs) from Apicomplexa. Calcium binding triggered a large conformational change that constituted a new mechanism in calcium signaling and a novel EF-hand fold (CAD, for CDPK activation domain). Thus we set out to determine if this mechanism was universal to all CDPKs. We solved additional CDPK structures, including one from the species Plasmodium. We highlight the similarities in sequence and structure across apicomplexan and plant CDPKs, and strengthen our observations that this novel mechanism could be universal to canonical CDPKs. Our new structures demonstrate more detailed steps in the mechanism of calcium activation and possible key players in regulation. Residues involved in making the largest conformational change are the most conserved across Apicomplexa, leading us to propose that the mechanism is indeed conserved. CpCDPK3_CAD and PfCDPK_CAD were captured at a possible intermediate conformation, lending insight into the order of activation steps. PfCDPK3_CAD adopts an activated fold, despite having an inactive EF-hand sequence in the N-terminal lobe. We propose that for most apicomplexan CDPKs, the mode of activation will be similar to that seen in our structures, while specific regulation of the inactive and active forms will require further investigation.

Tumor suppressor death-associated protein kinase is required for full IL-1β production

Interleukin-1β (IL-1β) is critical for inflammation and control of infection. The production of IL-1β depends on expression of pro-IL-1β and inflammasome component induced by inflammatory stimuli, followed by assembly of inflammasome to generate caspase-1 for cleavage of pro-IL-1β. Here we show that tumor suppressor death-associated protein kinase (DAPK) deficiency impaired IL-1β production in macrophages. Generation of tumor necrosis factor-α in macrophages, in contrast, was not affected by DAPK knockout. Two tiers of defects in IL-1β generation were found in DAPK-deficient macrophages: decreased pro-IL-1β induction by some stimuli and reduced caspase-1 activation by all inflammatory stimuli examined. With a normal NLRP3 induction in DAPK-deficient macrophages, the diminished caspase-1 generation is attributed to impaired inflammasome assembly. There is a direct binding of DAPK to NLRP3, suggesting an involvement of DAPK in inflammasome formation. We further illustrated that the formation of NLRP3 inflammasome in situ induced by inflammatory signals was impaired by DAPK deficiency. Taken together, our results identify DAPK as a molecule required for full production of IL-1β and functional assembly of the NLRP3 inflammasome. In addition, DAPK knockout reduced uric acid crystal-triggered peritonitis, suggesting that DAPK may serve as a target in the treatment of IL-1β-associated autoinflammatory diseases.

Discovery of novel PTP1B inhibitors with antihyperglycemic activity

AIM

To discover and optimize a series of novel PTP1B inhibitors containing a thiazolidinone-substituted biphenyl scaffold and to further evaluate the inhibitory effects of these compounds in vitro and in vivo.

METHODS

A total of 36 thiazolidinone substituted biphenyl scaffold derivatives were prepared. An in vitro biological evaluation was done by Enzyme-based assay. The in vivo efficacy of 7Fb as an antihyperglycemic agent was evaluated in a BKS db/db diabetic mouse model with a dose of 50 mg.kg(-1).d(-1) for 4 weeks.

RESULTS

The in vitro biological evaluation showed that compounds 7Fb and 7Fc could increase the insulin-induced tyrosine phosphorylation of IRbeta in CHO/hIR cells. In in vivo experiments, compound 7Fb significantly lowered the postprandial blood glucose, from 29.4+/-1.2 mmol/L with the vehicle to 24.7+/-0.6 mmol/L (P<0.01), and the fasting blood glucose from 27.3+/-1.5 mmol/L with the vehicle to 23.6+/-1.2 mmol/L (P<0.05).

CONCLUSION

A novel series of compounds were discovered to be PTP1B inhibitors. Among them, compound 7Fb significantly lowered the postprandial and fasting glucose levels, and the blood glucose level declined more rapidly than in metformin-treated mice. Thus, 7Fb may be a potential lead compound for developing new agents for the treatment of type II diabetes.

The Cullin 3 substrate adaptor KLHL20 mediates DAPK ubiquitination to control interferon responses

Death-associated protein kinase (DAPK) was identified as a mediator of interferon (IFN)-induced cell death. How IFN controls DAPK activation remains largely unknown. Here, we identify the BTB-Kelch protein KLHL20 as a negative regulator of DAPK. KLHL20 binds DAPK and Cullin 3 (Cul3) via its Kelch-repeat domain and BTB domain, respectively. The KLHL20-Cul3-ROC1 E3 ligase complex promotes DAPK polyubiquitination, thereby inducing the proteasomal degradation of DAPK. Accordingly, depletion of KLHL20 diminishes DAPK ubiquitination and degradation. The KLHL20-mediated DAPK ubiquitination is suppressed in cells receiving IFN-alpha or IFN-gamma, which induces an enrichment/sequestration of KLHL20 in the PML nuclear bodies, thereby separating KLHL20 from DAPK. Consequently, IFN triggers the stabilization of DAPK. This mechanism of DAPK stabilization is crucial for determining IFN responsiveness of tumor cells and contributes to IFN-induced autophagy. This study identifies KLHL20-Cul3-ROC1 as an E3 ligase for DAPK ubiquitination and reveals a regulatory mechanism of DAPK, through blocking its accessibility to this E3 ligase, in IFN-induced apoptotic and autophagic death. Our findings may be relevant to the problem of IFN resistance in cancer therapy.