NEAP causes down-regulation of EGFR, subsequently induces the suppression of NGF-induced differentiation in PC12 cells

More Than Meets the Eye: Revisiting the Roles of Heat Shock Factor 4 in Health and Diseases

Cells encounter a myriad of endogenous and exogenous stresses that could perturb cellular physiological processes. Therefore, cells are equipped with several adaptive and stress-response machinery to overcome and survive these insults. One such machinery is the heat shock response (HSR) program that is governed by the heat shock factors (HSFs) family in response towards elevated temperature, free radicals, oxidants, and heavy metals. HSF4 is a member of this HSFs family that could exist in two predominant isoforms, either the transcriptional repressor HSFa or transcriptional activator HSF4b. HSF4 is constitutively active due to the lack of oligomerization negative regulator domain. HSF4 has been demonstrated to play roles in several physiological processes and not only limited to regulating the classical heat shock- or stress-responsive transcriptional programs. In this review, we will revisit and delineate the recent updates on HSF4 molecular properties. We also comprehensively discuss the roles of HSF4 in health and diseases, particularly in lens cell development, cataract formation, and cancer pathogenesis. Finally, we will posit the potential direction of HSF4 future research that could enhance our knowledge on HSF4 molecular networks as well as physiological and pathophysiological functions.

A Review of DUSP26: Structure, Regulation and Relevance in Human Disease

Dual specificity phosphatases (DUSPs) play a crucial role in the regulation of intracellular signalling pathways, which in turn influence a broad range of physiological processes. DUSP malfunction is increasingly observed in a broad range of human diseases due to deregulation of key pathways, most notably the MAP kinase (MAPK) cascades. Dual specificity phosphatase 26 (DUSP26) is an atypical DUSP with a range of physiological substrates including the MAPKs. The residues that govern DUSP26 substrate specificity are yet to be determined; however, recent evidence suggests that interactions with a binding partner may be required for DUSP26 catalytic activity. DUSP26 is heavily implicated in cancer where, akin to other DUSPs, it displays both tumour-suppressive and -promoting properties, depending on the context. Here we review DUSP26 by evaluating its transcriptional patterns, protein crystallographic structure and substrate binding, as well as its physiological role(s) and binding partners, its role in human disease and the development of DUSP26 inhibitors.

MYCN amplification and ATRX mutations are incompatible in neuroblastoma

Aggressive cancers often have activating mutations in growth-controlling oncogenes and inactivating mutations in tumor-suppressor genes. In neuroblastoma, amplification of the MYCN oncogene and inactivation of the ATRX tumor-suppressor gene correlate with high-risk disease and poor prognosis. Here we show that ATRX mutations and MYCN amplification are mutually exclusive across all ages and stages in neuroblastoma. Using human cell lines and mouse models, we found that elevated MYCN expression and ATRX mutations are incompatible. Elevated MYCN levels promote metabolic reprogramming, mitochondrial dysfunction, reactive-oxygen species generation, and DNA-replicative stress. The combination of replicative stress caused by defects in the ATRX-histone chaperone complex, and that induced by MYCN-mediated metabolic reprogramming, leads to synthetic lethality. Therefore, ATRX and MYCN represent an unusual example, where inactivation of a tumor-suppressor gene and activation of an oncogene are incompatible. This synthetic lethality may eventually be exploited to improve outcomes for patients with high-risk neuroblastoma.

Dual-Specificity Phosphatases in Neuroblastoma Cell Growth and Differentiation

Dual-specificity phosphatases (DUSPs) are important regulators of neuronal cell growth and differentiation by targeting proteins essential to neuronal survival in signaling pathways, among which the MAP kinases (MAPKs) stand out. DUSPs include the MAPK phosphatases (MKPs), a family of enzymes that directly dephosphorylate MAPKs, as well as the small-size atypical DUSPs, a group of low molecular-weight enzymes which display more heterogeneous substrate specificity. Neuroblastoma (NB) is a malignancy intimately associated with the course of neuronal and neuroendocrine cell differentiation, and constitutes the source of more common extracranial solid pediatric tumors. Here, we review the current knowledge on the involvement of MKPs and small-size atypical DUSPs in NB cell growth and differentiation, and discuss the potential of DUSPs as predictive biomarkers and therapeutic targets in human NB.

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.

NEAP/DUSP26 suppresses receptor tyrosine kinases and regulates neuronal development in zebrafish

Expression of neuroendocrine-associated phosphatase (NEAP, also named as dual specificity phosphatase 26, [DUSP26]) is restricted to neuroendocrine tissues. We found that NEAP, but not its phosphatase-defective mutant, suppressed nerve growth factor (NGF) receptor TrkA and fibroblast growth factor receptor 1 (FGFR1) activation in PC12 cells upon NGF stimulation. Conversely, suppressing NEAP expression by RNA interference enhanced TrkA and FGFR1 phosphorylation. NEAP was capable of de-phosphorylating TrkA and FGFR1 directly in vitro. NEAP-orthologous gene existed in zebrafish. Morpholino (MO) suppression of NEAP in zebrafish resulted in hyper-phosphorylation of TrkA and FGFR1 as well as abnormal body postures and small eyes. Differentiation of retina in zebrafishes with NEAP MO treatment was severely defective, so were cranial motor neurons. Taken together, our data indicated that NEAP/DUSP26 have a critical role in regulating TrkA and FGFR1 signaling as well as proper development of retina and neuronal system in zebrafish.

Oncogenic MCT-1 activation promotes YY1-EGFR-MnSOD signaling and tumor progression

Tumor cells often produce high levels of reactive oxygen species (ROS) and display an increased ROS scavenging system. However, the molecular mechanism that balances antioxidative and oxidative stress in cancer cells is unclear. Here, we determined that oncogenic multiple copies in T-cell malignancy 1 (MCT-1) activity promotes the generation of intracellular ROS and mitochondrial superoxide. Overexpression of MCT-1 suppresses p53 accumulation but elevates the manganese-dependent superoxide dismutase (MnSOD) level via the YY1-EGFR signaling cascade, which protects cells against oxidative damage. Conversely, restricting ROS generation and/or targeting YY1 in lung cancer cells effectively inhibits the EGFR-MnSOD signaling pathway and cell invasiveness induced by MCT-1. Significantly, MCT-1 overexpression in lung cancer cells promotes tumor progression, necrosis and angiogenesis, and increases the number of tumor-promoting M2 macrophages and cancer-associated fibroblasts in the microenvironment. Clinical evidence further confirms that high expression of MCT-1 is associated with an increase in YY1, EGFR and MnSOD expression, accompanied by tumor recurrence, poor overall survival and EGFR mutation status in patients with lung cancers. Together, these data indicate that the MCT-1 oncogenic pathway is implicated in oxidative metabolism and lung carcinogenesis.

Structural Insight into the Critical Role of the N-Terminal Region in the Catalytic Activity of Dual-Specificity Phosphatase 26

Human dual-specificity phosphatase 26 (DUSP26) is a novel target for anticancer therapy because its dephosphorylation of the p53 tumor suppressor regulates the apoptosis of cancer cells. DUSP26 inhibition results in neuroblastoma cell cytotoxicity through p53-mediated apoptosis. Despite the previous structural studies of DUSP26 catalytic domain (residues 61-211, DUSP26-C), the high-resolution structure of its catalytically active form has not been resolved. In this study, we determined the crystal structure of a catalytically active form of DUSP26 (residues 39-211, DUSP26-N) with an additional N-terminal region at 2.0 Å resolution. Unlike the C-terminal domain-swapped dimeric structure of DUSP26-C, the DUSP26-N (C152S) monomer adopts a fold-back conformation of the C-terminal α8-helix and has an additional α1-helix in the N-terminal region. Consistent with the canonically active conformation of its protein tyrosine phosphate-binding loop (PTP loop) observed in the structure, the phosphatase assay results demonstrated that DUSP26-N has significantly higher catalytic activity than DUSP26-C. Furthermore, size exclusion chromatography-multiangle laser scattering (SEC-MALS) measurements showed that DUSP26-N (C152S) exists as a monomer in solution. Notably, the crystal structure of DUSP26-N (C152S) revealed that the N-terminal region of DUSP26-N (C152S) serves a scaffolding role by positioning the surrounding α7-α8 loop for interaction with the PTP-loop through formation of an extensive hydrogen bond network, which seems to be critical in making the PTP-loop conformation competent for phosphatase activity. Our study provides the first high-resolution structure of a catalytically active form of DUSP26, which will contribute to the structure-based rational design of novel DUSP26-targeting anticancer therapeutics.

Dual-specificity phosphatase 26 (DUSP26) stimulates Aβ42 generation by promoting amyloid precursor protein axonal transport during hypoxia

Amyloid beta peptide (Aβ) is a pathological hallmark of Alzheimer's disease (AD) and is generated through the sequential cleavage of amyloid precursor protein (APP) by β- and γ-secretases. Hypoxia is a known risk factor for AD and stimulates Aβ generation by γ-secretase; however, the underlying mechanisms remain unclear. In this study, we showed that dual-specificity phosphatase 26 (DUSP26) regulates Aβ generation through changes in subcellular localization of the γ-secretase complex and its substrate C99 under hypoxic conditions. DUSP26 was identified as a novel γ-secretase regulator from a genome-wide functional screen using a cDNA expression library. The phosphatase activity of DUSP26 was required for the increase in Aβ42 generation through γ-secretase, but this regulation did not affect the amount of the γ-secretase complex. Interestingly, DUSP26 induced the accumulation of C99 in the axons by stimulating anterograde transport of C99-positive vesicles. Additionally, DUSP26 induced c-Jun N-terminal kinase (JNK) activation for APP processing and axonal transport of C99. Under hypoxic conditions, DUSP26 expression levels were elevated together with JNK activation, and treatment with JNK inhibitor SP600125, or the DUSP26 inhibitor NSC-87877, reduced hypoxia-induced Aβ generation by diminishing vesicle trafficking of C99 to the axons. Finally, we observed enhanced DUSP26 expression and JNK activation in the hippocampus of AD patients. Our results suggest that DUSP26 mediates hypoxia-induced Aβ generation through JNK activation, revealing a new regulator of γ-secretase-mediated APP processing under hypoxic conditions. We propose the role of phosphatase dual-specificity phosphatase 26 (DUSP26) in the selective regulation of Aβ42 production in neuronal cells under hypoxic stress. Induction of DUSP26 causes JNK-dependent shift in the subcellular localization of γ-secretase and C99 from the cell body to axons for Aβ42 generation. These findings provide a new strategy for developing new therapeutic targets to arrest AD progression.

Dual-specificity phosphatases as molecular targets for inhibition in human disease

SIGNIFICANCE

The dual-specificity phosphatases (DUSPs) constitute a heterogeneous group of cysteine-based protein tyrosine phosphatases, whose members exert a pivotal role in cell physiology by dephosphorylation of phosphoserine, phosphothreonine, and phosphotyrosine residues from proteins, as well as other non-proteinaceous substrates.

RECENT ADVANCES

A picture is emerging in which a selected group of DUSP enzymes display overexpression or hyperactivity that is associated with human disease, especially human cancer, making feasible targeted therapy approaches based on their inhibition. A panoply of molecular and functional studies on DUSPs have been performed in the previous years, and drug-discovery efforts are ongoing to develop specific and efficient DUSP enzyme inhibitors. This review summarizes the current status on inhibitory compounds targeting DUSPs that belong to the MAP kinase phosphatases-, small-sized atypical-, and phosphatases of regenerating liver subfamilies, whose inhibition could be beneficial for the prevention or mitigation of human disease.

CRITICAL ISSUES

Achieving specificity, potency, and bioavailability are the major challenges in the discovery of DUSP inhibitors for the clinics. Clinical validation of compounds or alternative inhibitory strategies of DUSP inhibition has yet to come.

FUTURE DIRECTIONS

Further work is required to understand the dual role of many DUSPs in human cancer, their function-structure properties, and to identify their physiologic substrates. This will help in the implementation of therapies based on DUSPs inhibition.

The DUSP26 phosphatase activator adenylate kinase 2 regulates FADD phosphorylation and cell growth

Adenylate kinase 2 (AK2), which balances adenine nucleotide pool, is a multi-functional protein. Here we show that AK2 negatively regulates tumour cell growth. AK2 forms a complex with dual-specificity phosphatase 26 (DUSP26) phosphatase and stimulates DUSP26 activity independently of its AK activity. AK2/DUSP26 phosphatase protein complex dephosphorylates fas-associated protein with death domain (FADD) and regulates cell growth. AK2 deficiency enhances cell proliferation and induces tumour formation in a xenograft assay. This anti-growth function of AK2 is associated with its DUSP26-stimulating activity. Downregulation of AK2 is frequently found in tumour cells and human cancer tissues showing high levels of phospho-FADD(Ser194). Moreover, reconstitution of AK2 in AK2-deficient tumour cells retards both cell proliferation and tumourigenesis. Consistent with this, AK2(+/-) mouse embryo fibroblasts exhibit enhanced cell proliferation with a significant alteration in phospho-FADD(Ser191). These results suggest that AK2 is an associated activator of DUSP26 and suppresses cell proliferation by FADD dephosphorylation, postulating AK2 as a negative regulator of tumour growth.

Targeting DUSPs in glioblastomas - wielding a double-edged sword?

Several dual-specificity phosphatases (DUSPs) that play key roles in the direct or indirect inactivation of different MAP kinases (MAPKs) have been implicated in human cancers over the past decade. This has led to a growing interest in identifying DUSPs and their specific inhibitors for further testing and validation as therapeutic targets in human cancers. However, the lack of understanding of the complex regulatory mechanisms and cross-talks between MAPK signaling pathways, combined with the fact that DUSPs can act as a double-edged sword in cancer progression, calls for a more careful and thorough investigation. Among the various types of brain cancer, glioblastoma multiforme (GBM) is notorious for its aggressiveness and resistance to current treatment modalities. This has led to the search for new molecular targets, particularly those involving various signaling pathways. DUSPs appear to be a promising target, but much more information on DUSP targets and their effects on GBM is needed before potential therapies can be developed, tested, and validated. This review identifies and summarize the specific roles of DUSP1, DUSP4, DUSP6 and DUSP26 that have been implicated in GBM.

High-resolution crystal structure of the catalytic domain of human dual-specificity phosphatase 26

Dual-specificity phosphatases (DUSPs) play an important role in regulating cellular signalling pathways governing cell growth, differentiation and apoptosis. Human DUSP26 inhibits the apoptosis of cancer cells by dephosphorylating substrates such as p38 and p53. High-resolution crystal structures of the DUSP26 catalytic domain (DUSP26-C) and its C152S mutant [DUSP26-C (C152S)] have been determined at 1.67 and 2.20 Å resolution, respectively. The structure of DUSP26-C showed a novel type of domain-swapped dimer formed by extensive crossover of the C-terminal α7 helix. Taken together with the results of a phosphatase-activity assay, structural comparison with other DUSPs revealed that DUSP26-C adopts a catalytically inactive conformation of the protein tyrosine phosphate-binding loop which significantly deviates from that of canonical DUSP structures. In particular, a noticeable difference exists between DUSP26-C and the active forms of other DUSPs at the hinge region of a swapped C-terminal domain. Additionally, two significant gaps were identified between the catalytic core and its surrounding loops in DUSP26-C, which can be exploited as additional binding sites for allosteric enzyme regulation. The high-resolution structure of DUSP26-C may thus provide structural insights into the rational design of DUSP26-targeted anticancer drugs.

Luteolin induces microRNA-132 expression and modulates neurite outgrowth in PC12 cells

Luteolin (3′,4′,5,7-tetrahydroxyflavone), a food-derived flavonoid, has been reported to exert neurotrophic properties that are associated with its capacity to promote neuronal survival and neurite outgrowth. In this study, we report for the first time that luteolin induces the persistent expression of microRNA-132 (miR-132) in PC12 cells. The correlation between miR-132 knockdown and a decrease in luteolin-mediated neurite outgrowth may indicate a mechanistic link by which miR-132 functions as a mediator for neuritogenesis. Furthermore, we find that luteolin led to the phosphorylation and activation of cAMP response element binding protein (CREB), which is associated with the up-regulation of miR-132 and neurite outgrowth. Moreover, luteolin-induced CREB activation, miR-132 expression and neurite outgrowth were inhibited by adenylate cyclase, protein kinase A (PKA) and MAPK/ERK kinase 1/2 (MEK1/2) inhibitors but not by protein kinase C (PKC) or calcium/calmodulin-dependent protein kinase II (CaMK II) inhibitors. Consistently, we find that luteolin treatment increases ERK phosphorylation and PKA activity in PC12 cells. These results show that luteolin induces the up-regulation of miR-132, which serves as an important regulator for neurotrophic actions, mainly acting through the activation of cAMP/PKA- and ERK-dependent CREB signaling pathways in PC12 cells.

JNK pathway-associated phosphatase dephosphorylates focal adhesion kinase and suppresses cell migration

JNK pathway-associated phosphatase (JKAP, also named DUSP22) is expressed in various tissues, indicating that JKAP may have an important biological function. We showed that JKAP localized in the actin filament-enriched region. Expression of JKAP reduced cell migration, whereas a JKAP mutant lacking catalytic activity promoted cell motility. JKAP efficiently removed tyrosine phosphorylation of several proteins. We have identified focal adhesion kinase (FAK) as a substrate of JKAP. Overexpression of JKAP, but not JKAP mutant lacking catalytic activity, decreased FAK phosphorylation at tyrosines 397, 576, and 577 in H1299 cells. Consistent with these results, decreasing JKAP expression by RNA interference promoted cell migration and Src-induced FAK phosphorylation. Taken together, this study identified a new role for JKAP in the modulation of FAK phosphorylation and cell motility.