Dual-specificity phosphatase Pyst2-L is constitutively highly expressed in myeloid leukemia and other malignant cells

Knockdown of dual-specificity phosphatase 3 drives differentiation and polarization of myeloid leukemia cells into macrophages with reduced proliferative and DNA repair fitness

Dual-specificity phosphatase 3 (DUSP3) regulates key cellular processes, including the cell cycle, proliferation, and differentiation. Recently, we demonstrated its crucial role in maintaining genomic stability by interacting with and dephosphorylating nucleophosmin (NPM), thereby modulating nuclear p53 activity under genotoxic stress. Given the frequent mutations in both p53 and NPM in acute myeloid leukemia (AML), this study aimed to investigate the impact of DUSP3 knockdown in two p53-deficient AML cell lines and explore potential correlations with NPM expression. THP-1 cells exhibited higher basal levels of DUSP3 and NPM compared to HL-60 cells, while DUSP3 knockdown reduced NPM expression in HL-60 cells. Upon phorbol 12-myristate 13-acetate (PMA)-induced differentiation into macrophage-like cells, only HL-60 cells displayed decreased levels of both DUSP3 and NPM. DUSP3 knockdown enhanced differentiation in THP-1 and HL-60 cells and promoted non-classical M2 macrophage polarization following additional PMA exposure, as indicated by increased expression of CD11b and CD206. Bioinformatics analysis revealed significant correlations between DUSP3 and NPM gene expression, AML patient survival, and the maturation stage of myeloid cells. Furthermore, DUSP3 knockdown in undifferentiated HL-60 cells impaired proliferation and compromised genomic stability under genotoxic stress induced by doxorubicin. These findings suggest that DUSP3 plays a regulatory role in the differentiation, polarization, and proliferation of myeloid cells. Through the modulation of NPM expression and activity, DUSP3 may contribute to a deeper understanding of leukemia pathophysiology and mechanisms of chemotherapy resistance.

Mitogen-Activated Protein Kinase Phosphatases: No Longer Undruggable?

Phosphatases and kinases maintain an equilibrium of dephosphorylated and phosphorylated proteins, respectively, that are required for critical cellular functions. Imbalance in this equilibrium or irregularity in their function causes unfavorable cellular effects that have been implicated in the development of numerous diseases. Protein tyrosine phosphatases (PTPs) catalyze the dephosphorylation of protein substrates on tyrosine residues, and their involvement in cell signaling and diseases such as cancer and inflammatory and metabolic diseases has made them attractive therapeutic targets. However, PTPs have proved challenging in therapeutics development, garnering them the unfavorable reputation of being undruggable. Nonetheless, great strides have been made toward the inhibition of PTPs over the past decade. Here, we discuss the advancement in small-molecule inhibition for the PTP subfamily known as the mitogen-activated protein kinase (MAPK) phosphatases (MKPs). We review strategies and inhibitor discovery tools that have proven successful for small-molecule inhibition of the MKPs and discuss what the future of MKP inhibition potentially might yield.

Dual specificity phosphatase 7 drives the formation of cardiac mesoderm in mouse embryonic stem cells

Dual specificity phosphatase 7 (DUSP7) is a protein belonging to a broad group of phosphatases that can dephosphorylate phosphoserine/phosphothreonine as well as phosphotyrosine residues within the same substrate. DUSP7 has been linked to the negative regulation of mitogen activated protein kinases (MAPK), and in particular to the regulation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). MAPKs play an important role in embryonic development, where their duration, magnitude, and spatiotemporal activity must be strictly controlled by other proteins, among others by DUSPs. In this study, we focused on the effect of DUSP7 depletion on the in vitro differentiation of mouse embryonic stem (ES) cells. We showed that even though DUSP7 knock-out ES cells do retain some of their basic characteristics, when it comes to differentiation, they preferentially differentiate towards neural cells, while the formation of early cardiac mesoderm is repressed. Therefore, our data indicate that DUSP7 is necessary for the correct formation of neuroectoderm and cardiac mesoderm during the in vitro differentiation of ES cells.

DUSP7 inhibits cervical cancer progression by inactivating the RAS pathway

To determine the differentially expressed proteins (DEPs) between paired samples of cervical cancer (CC) and paracancerous tissue by quantitative proteomics and to examine the effects of DUSP7 expression on the tumorigenesis and progression of CC. Proteomic profiles of three paired samples of CC and paracancerous tissue were quantitatively analysed to identify DEPs. The relationship between DEP expression and patient clinicopathological characteristics and prognosis was evaluated. The effects of the selected DEPs on CC progression were examined in SIHA cells. A total of 129 DEPs were found. Western blot and immunohistochemistry (IHC) staining analyses confirmed the results from quantitative proteomic analysis showing that the selected DEP, HRAS, P-ERK1/2, and PLD1 levels were increased, whereas the DUSP7 level was decreased in CC tissue compared with the paired normal paracancerous tissues. The IHC results from the CC TMA analysis showed that the decreased expression of DUSP7 (p = 0.045 and 0.044) was significantly associated with a tumour size >2 cm and parametrial infiltration. In addition, the decreased expression of DUSP7 and increased expression of p-ERK1/2 were adversely related to patient relapse (p = 0.003 and 0.001) and survival (p = 0.034 and 0.006). The expression of HRAS and p-ERK1/2 was decreased in DUSP7-SIHA cells compared with NC-SIHA cells (p = 0.0003 and 0.0026). Biological functions in vitro, including invasion, migration and proliferation and tumour formation in vivo were decreased in DUSP7-SIHA cells (all p < 0.05) but increased in shDUSP7-SIHA cells (all p < 0.05). DUSP7 inhibits cervical cancer progression by inactivating the RAS pathway.

Dysregulation of Dual-Specificity Phosphatases by Epstein-Barr Virus LMP1 and Its Impact on Lymphoblastoid Cell Line Survival

The strongest evidence of the oncogenicity of Epstein-Barr virus (EBV) in vitro is its ability to immortalize human primary B lymphocytes into lymphoblastoid cell lines (LCLs). Yet the underlying mechanisms explaining how the virus tempers the growth program of the host cells have not been fully elucidated. The mitogen-activated protein kinases (MAPKs) are implicated in many cellular processes and are constitutively activated in LCLs. We questioned the expression and regulation of the dual-specificity phosphatases (DUSPs), the main negative regulator of MAPKs, during EBV infection and immortalization. Thirteen DUSPs, including 10 typical and 3 atypical types of DUSPs, were tested. Most of them were downregulated after EBV infection. Here, a role of viral oncogene latent membrane protein 1 (LMP1) in limiting DUSP6 and DUSP8 expression was identified. Using MAPK inhibitors, we found that LMP1 activates extracellular signal-regulated kinase (ERK) or p38 to repress the expression of DUSP6 and DUSP8, with corresponding substrate specificity. Morphologically, overexpression of DUSP6 and DUSP8 attenuates the ability of EBV-immortalized LCL cells to clump together. Mechanistically, apoptosis induced by restoring DUSP6 and DUSP8 in LCLs indicated a novel mechanism for LMP1 to provide a survival signal during EBV immortalization. Collectively, this report provides the first description of the interplay between EBV genes and DUSPs and contributes considerably to the interpretation of MAPK regulation in EBV immortalization.IMPORTANCE Infections by the ubiquitous Epstein-Barr virus (EBV) are associated with a wide spectrum of lymphomas and carcinomas. It has been well documented that activation levels of MAPKs are found in cancer cells to translate various external or intrinsic stimuli into cellular responses. Physiologically, the dual-specificity phosphates (DUSPs) exhibit great ability in regulating MAPK activities with respect to their capability of dephosphorylating MAPKs. In this study, we found that DUSPs were generally downregulated after EBV infection. EBV oncogenic latent membrane protein 1 (LMP1) suppressed DUSP6 and DUSP8 expression via MAPK pathway. In this way, LMP1-mediated MAPK activation was a continuous process. Furthermore, DUSP downregulation was found to contribute greatly to prevent apoptosis of EBV-infected cells. To sum up, this report sheds light on a novel molecular mechanism explaining how EBV maintains the unlimited proliferation status of the immortalized cells and provides a new link to understand EBV-induced B cell survival.

Protein tyrosine phosphatases: promising targets in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is the most common type of pancreatic cancer. It is the fourth leading cause of cancer-related death and is associated with a very poor prognosis. KRAS driver mutations occur in approximately 95% of PDAC cases and cause the activation of several signaling pathways such as mitogen-activated protein kinase (MAPK) pathways. Regulation of these signaling pathways is orchestrated by feedback loops mediated by the balance between protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs), leading to activation or inhibition of its downstream targets. The human PTPome comprises 125 members, and these proteins are classified into three distinct families according to their structure. Since PTP activity description, it has become clear that they have both inhibitory and stimulatory effects on cancer-associated signaling processes and that deregulation of PTP function is closely associated with tumorigenesis. Several PTPs have displayed either tumor suppressor or oncogenic characteristics during the development and progression of PDAC. In this sense, PTPs have been presented as promising candidates for the treatment of human pancreatic cancer, and many PTP inhibitors have been developed since these proteins were first associated with cancer. Nevertheless, some challenges persist regarding the development of effective and safe methods to target these molecules and deliver these drugs. In this review, we discuss the role of PTPs in tumorigenesis as tumor suppressor and oncogenic proteins. We have focused on the differential expression of these proteins in PDAC, as well as their clinical implications and possible targeting for pharmacological inhibition in cancer therapy.

Long non-coding RNA MIAT promotes breast cancer progression and functions as ceRNA to regulate DUSP7 expression by sponging miR-155-5p

Long non-coding RNAs (lncRNA) have been reported as key regulators in the progression and metastasis of breast cancer. In this study, we found that the lncRNA myocardial infarction associated transcript (MIAT) expression was upregulated in breast cancer in The Cancer Genome Atlas (TCGA) data sets. We validated that MIAT was higher in breast cancer cell lines and advanced breast tumors than in normal controls. And MIAT overexpression associated with TNM stage and lymphnode metastasis. Knockdown MIAT inhibited breast cancer cell proliferation and promoted apoptosis. Also MIAT downregulation suppressed epithelial-mesenchymal transition (EMT) and decreased migration and invasion in MDA-MB-231 and MCF-7 breast cancer cell lines. More importantly, knockdown MIAT inhibited tumor growth in vivo. Our results suggested that MIAT acted as a competing endogenous RNA (ceRNA) to regulate the expression of dual specificity phosphatase 7 (DUSP7) by taking up miR-155-5p in breast cancer. There were positive correlation between MIAT and DUSP7 expression in breast cancer patients. We conclude that MIAT promotes breast cancer progression and functions as ceRNA to regulate DUSP7 expression by sponging miR-155-5p in breast cancer.

Regulatory Roles of MAPK Phosphatases in Cancer

The mitogen-activated protein kinases (MAPKs) are key regulators of cell growth and survival in physiological and pathological processes. Aberrant MAPK signaling plays a critical role in the development and progression of human cancer, as well as in determining responses to cancer treatment. The MAPK phosphatases (MKPs), also known as dual-specificity phosphatases (DUSPs), are a family of proteins that function as major negative regulators of MAPK activities in mammalian cells. Studies using mice deficient in specific MKPs including MKP1/DUSP1, PAC-1/DUSP2, MKP2/DUSP4, MKP5/DUSP10 and MKP7/DUSP16 demonstrated that these molecules are important not only for both innate and adaptive immune responses, but also for metabolic homeostasis. In addition, the consequences of the gain or loss of function of the MKPs in normal and malignant tissues have highlighted the importance of these phosphatases in the pathogenesis of cancers. The involvement of the MKPs in resistance to cancer therapy has also gained prominence, making the MKPs a potential target for anti-cancer therapy. This review will summarize the current knowledge of the MKPs in cancer development, progression and treatment outcomes.

Mutations in tyrosine kinase and tyrosine phosphatase and their relevance to the target therapy in hematologic malignancies

Protein tyrosine kinases and protein tyrosine phosphatases play pivotal roles in regulation of cellular phosphorylation and signal transduction with opposite functions. Accumulating evidences have uncovered the relevance of genetic alterations in these two family members to hematologic malignancies. This review underlines progress in understanding the pathogenesis of these genetic alterations including mutations and aberrant expression and the evolving protein tyrosine kinases and protein tyrosine phosphatases targeted therapeutic strategies in hematologic neoplasms.

Structure of human dual-specificity phosphatase 7, a potential cancer drug target

Human dual-specificity phosphatase 7 (DUSP7/Pyst2) is a 320-residue protein that belongs to the mitogen-activated protein kinase phosphatase (MKP) subfamily of dual-specificity phosphatases. Although its precise biological function is still not fully understood, previous reports have demonstrated that DUSP7 is overexpressed in myeloid leukemia and other malignancies. Therefore, there is interest in developing DUSP7 inhibitors as potential therapeutic agents, especially for cancer. Here, the purification, crystallization and structure determination of the catalytic domain of DUSP7 (Ser141-Ser289/C232S) at 1.67 Å resolution are reported. The structure described here provides a starting point for structure-assisted inhibitor-design efforts and adds to the growing knowledge base of three-dimensional structures of the dual-specificity phosphatase family.

A Drosophila-centric view of protein tyrosine phosphatases

Most of our knowledge on protein tyrosine phosphatases (PTPs) is derived from human pathologies and mouse knockout models. These models largely correlate well with human disease phenotypes, but can be ambiguous due to compensatory mechanisms introduced by paralogous genes. Here we present the analysis of the PTP complement of the fruit fly and the complementary view that PTP studies in Drosophila will accelerate our understanding of PTPs in physiological and pathological conditions. With only 44 PTP genes, Drosophila represents a streamlined version of the human complement. Our integrated analysis places the Drosophila PTPs into evolutionary and functional contexts, thereby providing a platform for the exploitation of the fly for PTP research and the transfer of knowledge onto other model systems.

Clustering nuclear receptors in liver regeneration identifies candidate modulators of hepatocyte proliferation and hepatocarcinoma

Background & Aims

Liver regeneration (LR) is a valuable model for studying mechanisms modulating hepatocyte proliferation. Nuclear receptors (NRs) are key players in the control of cellular functions, being ideal modulators of hepatic proliferation and carcinogenesis.

Methods & Results

We used a previously validated RT-qPCR platform to profile modifications in the expression of all 49 members of the NR superfamily in mouse liver during LR. Twenty-nine NR transcripts were significantly modified in their expression during LR, including fatty acid (peroxisome proliferator-activated receptors, PPARs) and oxysterol (liver X receptors, Lxrs) sensors, circadian masters RevErbα and RevErbβ, glucocorticoid receptor (Gr) and constitutive androxane receptor (Car). In order to detect the NRs that better characterize proliferative status vs. proliferating liver, we used the novel Random Forest (RF) analysis to selected a trio of down-regulated NRs (thyroid receptor alpha, Trα; farsenoid X receptor beta, Fxrβ; Pparδ) as best discriminators of the proliferating status. To validate our approach, we further studied PPARδ role in modulating hepatic proliferation. We first confirmed the suppression of PPARδ both in LR and human hepatocellular carcinoma at protein level, and then demonstrated that PPARδ agonist GW501516 reduces the proliferative potential of hepatoma cells.

Conclusions

Our data suggest that NR transcriptome is modulated in proliferating liver and is a source of biomarkers and bona fide pharmacological targets for the management of liver disease affecting hepatocyte proliferation.

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 family-wide structure and function of human dual-specificity protein phosphatases

Dual-specificity protein phosphatases (DUSPs), which dephosphorylate both phosphoserine/threonine and phosphotyrosine, play vital roles in immune activation, brain function and cell-growth signalling. A family-wide structural library of human DUSPs was constructed based on experimental structure determination supplemented with homology modelling. The catalytic domain of each individual DUSP has characteristic features in the active site and in surface-charge distribution, indicating substrate-interaction specificity. The active-site loop-to-strand switch occurs in a subtype-specific manner, indicating that the switch process is necessary for characteristic substrate interactions in the corresponding DUSPs. A comprehensive analysis of the activity-inhibition profile and active-site geometry of DUSPs revealed a novel role of the active-pocket structure in the substrate specificity of DUSPs. A structure-based analysis of redox responses indicated that the additional cysteine residues are important for the protection of enzyme activity. The family-wide structures of DUSPs form a basis for the understanding of phosphorylation-mediated signal transduction and the development of therapeutics.

Inside the human cancer tyrosine phosphatome

Members of the protein tyrosine phosphatase (Ptp) family dephosphorylate target proteins and counter the activities of protein tyrosine kinases that are involved in cellular phosphorylation and signalling. As such, certain PTPs might be tumour suppressors. Indeed, PTPs play an important part in the inhibition or control of growth, but accumulating evidence indicates that some PTPs may exert oncogenic functions. Recent large-scale genetic analyses of various human tumours have highlighted the relevance of PTPs either as putative tumour suppressors or as candidate oncoproteins. Progress in understanding the regulation and function of PTPs has provided insights into which PTPs might be potential therapeutic targets in human cancer.

Expression and functional role of negative signalling regulators in tumour development and progression

Alterations in intracellular signalling pathways such as the mitogen-activated protein kinases (MAPKs) are key common mechanisms of tumour development and progression. As such, there has been intense research into developing drugs that can inhibit or attenuate intracellular signalling. In recent years, there has been increasing recognition that the cell already has innate negative regulatory proteins that achieve this in normal homeostasis. These regulators provide a feedback inhibitory mechanism that controls the intensity and duration of activated signalling by exogenous stimuli. Members of this group include Raf kinase inhibitor protein 1, the MAPK phosphatases, the SPROUTY and SPRED families and similar expression to FGF. A number of studies have now demonstrated significant alterations in expression of negative regulators in malignant tissue in different cancer types. In functional studies, manipulated expression of these regulators has been shown to significantly influence tumour cell behaviour and phenotype. Here, we summarise the evidence for the functional expression of negative signalling regulators in tumour growth and progression and discuss their potential role as cancer biomarkers and targets for novel drug therapy.

Mitogen-activated protein (MAP) kinase/MAP kinase phosphatase regulation: roles in cell growth, death, and cancer

Mitogen-activated protein kinase dual-specificity phosphatase-1 (also called MKP-1, DUSP1, ERP, CL100, HVH1, PTPN10, and 3CH134) is a member of the threonine-tyrosine dual-specificity phosphatases, one of more than 100 protein tyrosine phosphatases. It was first identified approximately 20 years ago, and since that time extensive investigations into both mkp-1 mRNA and protein regulation and function in different cells, tissues, and organs have been conducted. However, no general review on the topic of MKP-1 exists. As the subject matter pertaining to MKP-1 encompasses many branches of the biomedical field, we focus on the role of this protein in cancer development and progression, highlighting the potential role of the mitogen-activated protein kinase (MAPK) family. Section II of this article elucidates the MAPK family cross-talk. Section III reviews the structure of the mkp-1 encoding gene, and the known mechanisms regulating the expression and activity of the protein. Section IV is an overview of the MAPK-specific dual-specificity phosphatases and their role in cancer. In sections V and VI, mkp-1 mRNA and protein are examined in relation to cancer biology, therapeutics, and clinical studies, including a discussion of the potential role of the MAPK family. We conclude by proposing an integrated scheme for MKP-1 and MAPK in cancer.

Dual-specificity MAP kinase phosphatases (MKPs) and cancer

There are ten mitogen-activated protein kinase (MAPK) phosphatases (MKPs) that act as negative regulators of MAPK activity in mammalian cells and these can be subdivided into three groups. The first comprises DUSP1/MKP-1, DUSP2/PAC1, DUSP4/MKP-2 and DUSP5/hVH-3, which are inducible nuclear phosphatases. With the exception of DUSP5, these MKPs display a rather broad specificity for inactivation of the ERK, p38 and JNK MAP kinases. The second group contains three closely related ERK-specific and cytoplasmic MKPs encoded by DUSP6/MKP-3, DUSP7/MKP-X and DUSP9/MKP-4. The final group consists of three MKPs DUSP8/hVH-5, DUSP10/MKP-5 and DUSP16/MKP-7 all of which preferentially inactivate the stress-activated p38 and JNK MAP kinases. Abnormal MAPK signalling will have important consequences for processes critical to the development and progression of human cancer. In addition, MAPK signalling also plays a key role in determining the response of tumour cells to conventional cancer therapies. The emerging roles of the dual-specificity MKPs in the regulation of MAPK activities in normal tissues has highlighted the possible pathophysiological consequences of either loss (or gain) of function of these enzymes as part of the oncogenic process. This review summarises the current evidence implicating the dual-specificity MKPs in the initiation and development of cancer and also on the outcome of treatment.

Expression of ERK signaling inhibitors Dusp6, Dusp7, and Dusp9 during mouse ear development

The levels of fibroblast growth factor (FGF) signaling play important roles in coordinating development of the mouse inner, middle, and outer ears. Extracellular signal-regulated kinases (ERKs) are among the effectors that transduce the FGF signal to the nucleus and other cellular compartments. Attenuation of ERK activity by dephosphorylation is necessary to modulate the magnitude and duration of the FGF signal. Recently, we showed that inactivation of the ERK phosphatase, dual specificity phosphatase 6 (DUSP6), causes partially penetrant postnatal lethality, hearing loss and skeletal malformations. To determine whether other Dusps may function redundantly with Dusp6 during otic development, we surveyed the expression domains of the three ERK-specific DUSP transcripts, Dusp6, Dusp7, and Dusp9, in the embryonic mouse ear. We show that each is expressed in partially overlapping patterns that correspond to regions of active FGF signaling, suggesting combinatorial roles in negative regulation of this pathway during ear development.

Role of mitogen-activated protein kinase phosphatases (MKPs) in cancer

The mitogen-activated protein kinase (MAPK) phosphatases (MKPs) are a family of dual-specificity protein phosphatases that dephosphorylate both phospho-threonine and phospho-tyrosine residues in MAP kinases, including the c-Jun N-terminal protein kinase (JNK)/stress-activated protein kinase (SAPK), the p38 MAPK, and the extracellular signal-related kinase (ERK). Since phosphorylation is required for the activation of MAP kinases, dephosphorylation by MKPs inhibits MAPK activity, thereby negatively regulating MAPK signaling. It is known that deregulation of MAPK signaling is the most common alteration in human cancers. Recent studies have suggested that MKPs play an important role not only in the development of cancers, but also in the response of cancer cells to chemotherapy. Thus, understanding the roles of MKPs in the development of cancer and their impact on chemotherapy can be exploited for therapeutic benefits for the treatment of human cancer.

Constitutive Fms-like tyrosine kinase 3 activation results in specific changes in gene expression in myeloid leukaemic cells

Constitutively activating internal tandem duplication (ITD) mutations of the receptor tyrosine kinase FLT3 (Fms-like tyrosine kinase 3) play an important role in leukaemogenesis. We have examined, by cDNA microarray analysis, the changes in gene expression induced by FLT3/ITD or constitutively activated wild type FLT3 signalling. A limited set of genes was consistently affected by FLT3 inhibition. In confirmation of their FLT3 dependence, these genes returned toward pretreatment levels of expression after reversal of FLT3 inhibition. Several of the most significantly affected genes are involved in the RAS/mitogen-activated protein kinase, Janus kinase/signal transducer and activator of transcription and phosphatidylinositol 3 kinase (PI3K)/AKT pathways. These data suggest that constitutively activated FLT3 works through multiple signal transduction pathways. PIM1, MYC and CCND3 were chosen from this gene set to explore their biological roles. Knock-down of these genes by small interfering RNA showed that these genes play important roles in constitutively activated FLT3 expressing cells. The alterations of the gene expression profiles in these cells help to further elucidate the mechanisms of FLT3-mediated leukaemogenesis.

Regulation of MAP kinases by MAP kinase phosphatases

MAP kinase phosphatases (MKPs) catalyze dephosphorylation of activated MAP kinase (MAPK) molecules and deactivate them. Therefore, MKPs play an important role in determining the magnitude and duration of MAPK activities. MKPs constitute a structurally distinct family of dual-specificity phosphatases. The MKP family members share the sequence homology and the preference for MAPK molecules, but they are different in substrate specificity among MAPK molecules, tissue distribution, subcellular localization and inducibility by extracellular stimuli. Our understanding of their protein structure, substrate recognition mechanisms, and regulatory mechanisms of the enzymatic activity has greatly increased over the past few years. Furthermore, although there are a number of MKPs, that have similar substrate specificities, non-redundant roles of MKPs have begun to be identified. Here we focus on recent findings regarding regulation and function of the MKP family members as physiological regulators of MAPK signaling.

Targeting dual-specificity phosphatases: manipulating MAP kinase signalling and immune responses

Dual-specificity phosphatases (DUSPs) are a subset of protein tyrosine phosphatases, many of which dephosphorylate threonine and tyrosine residues on mitogen-activated protein kinases (MAPKs), and hence are also referred to as MAPK phosphatases (MKPs). The regulated expression and activity of DUSP family members in different cells and tissues controls MAPK intensity and duration to determine the type of physiological response. For immune cells, DUSPs regulate responses in both positive and negative ways, and DUSP-deficient mice have been used to identify individual DUSPs as key regulators of immune responses. From a drug discovery perspective, DUSP family members are promising drug targets for manipulating MAPK-dependent immune responses in a cell-type and disease-context-dependent manner, to either boost or subdue immune responses in cancers, infectious diseases or inflammatory disorders.

The Pyst2-L phosphatase is involved in cell-crowding

The dual-specificity phosphatase Pyst2-L was found to be over expressed in leukocytes derived from AML and ALL patients as well as in certain other solid tumors and lymphoblastoid cell lines. Pyst2-L, binds and dephosphorylates both pERKs and pJNKs proteins, and thus, plays a role in regulating the MAP kinase signaling pathway. In the present study, a comparative genomic application was used and sequence analysis of multi-organisms databases were searched in order to identify genes homologous to Pyst2-L. The Xenopus laevis MAP kinase phosphatase X17c gene and the Yeast nitrogen starvation-induced protein phosphatase Yvh1p gene were revealed to be highly homologous with Pyst2-L. Both X17c and Yvh1p genes play a role in cell cycle regulation. A down regulated expression of the Yvh1p gene occurred in Saccharomyces cerevisiae that were synchronized to the G2-phase of the cell cycle by alpha-factor. In conformity with this result, a reduction in Pyst2-L expression levels was observed in G2-phase-synchronized Human K562 cells. Finally, we were able to show that cells in highly crowded cultures express high levels of the Pyst2-L phosphatase. These observations may indicate that low levels of the Pyst2-L phosphatase are essential for the G2-phase of the cell cycle and that this phosphatase might play a role in signaling cascades induced by cellular crowding.

MKP-8, a novel MAPK phosphatase that inhibits p38 kinase

Intracellular signaling pathways and their relationship to malignant progression have become a major focus of cancer biology. The dual-specificity phosphatase (DSP) family is a more recently identified family of intracellular signaling modulators. We have identified a novel protein phosphatase with a well-conserved DSP catalytic domain containing the DSP catalytic motif, xHCxxGxSRS, and mitogen-activated protein kinase phosphatase (MKP) motif, AYLM. Because of these unique characteristics, the protein was named mitogen-activated protein kinase phosphatase-8 (MKP-8). This protein is approximately 20kDa in size and mainly localizes to the nuclear compartment of the cell. MKP-8 is expressed in embryonal cancers (retinoblastoma, neuroepithelioma, and neuroblastoma) and has limited expression in normal tissues. MKP-8 displays significant phosphatase activity that is inhibited by a cysteine to serine substitution in the catalytic domain. When co-expressed with activated MAPKs, MKP-8 is able to inhibit p38 kinase phosphorylation and downstream activity.